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John Mahoney

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Jul-30-02, 06:17 PM (EST) |
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"Benjamin Banneker's Math Puzzles"
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I'm a high school math teacher at Benjamin Banneker Academic HS in Washington, DC. Briefly, I've written a paper on Benjamin Banneker's mathematics - based on his journal which is on microfilm in Baltimore. Banneker was an African American self taught mathematician and astronomer who lived in Maryland 200 years ago. He, in the late 18th century, wrote down mathematical puzzles. In the one volume that survived, there are about 6 of these. I figured out all but one of them. I'm sure that the problem with the one that I don't understand is that I'm not interpreting the meaning of what is written correctly. The solution to the puzzle is given within the puzzle and Banneker also wrote a proportion at the end which yields the correct solution. I've included below the puzzle - which is given in rhyme form (typical of the era), his solution, my interpretation of the meaning of key lines and my attempts at solutions - which aren't correct. I would appreciate any help you could give. Thanks, John Mahoney, mahoneyj@sidwell.edu Puzzle of the Dog and the Hare recorded by Benjamin Banneker in his Journal circa 1792 When fleecy skies have Cloth’d the ground With a white mantle all around Then with a grey hound Snowy fair In milk white fields we Cours’d a Hare Just in the midst of a Champaign We set her up, away she ran, The Hound I think was from her then Just Thirty leaps or three times ten Oh it was pleasant to see How the Hare did run so timorously But yet so very Swift that I Did think she did not run but Fly When the Dog was almost at her heels She quickly turn’d, and down the fields She ran again with full Career And ‘gain she turn’d to the place she were At every turn she gan’d of ground As many yards as the greyhound Could leap at thrice, and She did make, Just Six, if I do not mistake Four times She Leap’d for the dogs three But two of the Dogs leaps did agree With three of hers, nor pray declare How many leaps he took to Catch the Hare. (Answer) Just Seventy two I did Suppose, An Answer false from thence arose, I doubled the Sum of Seventy two, But still I found that would not do, I mix’d the Numbers of them both, Which Shew’d so plain that I’ll make Oath, Eight hundred leaps the Dog did make, And Sixty four, the Hare to take. The work that follows is equivalent to the proportion 4/24 = 48/x solved to find x = 864. Questions? What is the mathematical meaning of the lines of the puzzle? Why is 72 almost correct? Why is 864 correct? Where did the numbers 4 and 48 come from in the solution? Statements 1) Initially the dog is 30 dog leaps from the hare. 2) The hare turns when the dog reaches the hare. 3) The hare gains 3 dog leap’s worth of yards at each turn. 4) The hare makes 6 turns. 5) 4 times the number of hare leaps = 3 times the number of dog leaps. 6) 2 times the length of each dog leap = 3 times the length of each hare leap. 7) The question: How many leaps does the dog make in order to catch the hare? Statements 5) and 6) combine to state that the dog runs twice as fast as the hare. Solution attempt #1: Since initially the dog is 30 dog leaps from the hare and the dog runs twice as fast as the hare, then the dog reaches the hare in 60 dog leaps. At that instant the hare turns and is then three dog leaps ahead of the dog. The dog chases the hare and reaches the hare in 6 dog leaps. The hare turns again and, again, is three dog leaps ahead of the dog. This continues for a total of six turns until the dog finally catches the hare. Thus the dog has run 60 + 6*6 = 96 dog leaps. This answer is 1/9 of 864. (perhaps this solution is flawed because it relies on fractional leaps? Check this.) Solution attempt #2: This attempt is based on interpreting the phrase “turn’d to the place she were” to mean the hare returned to the place where the hare started and at that point was an additional 3 dog leap’s worth of yards ahead of the dog. Since initially the dog is 30 dog leaps from the hare and the dog runs twice as fast as the hare, then the dog reaches the hare in 60 dog leaps. At that instant the hare turns and is then 33 dog leaps ahead of the dog. The dog chases the hare and reaches the hare in 66 dog leaps. The hare turns again and, again, is 33 dog leaps ahead of the dog. This continues for a total of six turns until the dog finally catches the hare. Thus the dog has run 60 + 6*66 = 456 dog leaps. A variation of this solution has the hare at each turn becoming increasingly further ahead of the dog by three dog leaps at each turn. So the solution becomes, in dog leaps: 60 + 66 + 72 + 78 + 84 + 90 + 96 = 546 dog leaps. Note: Maybe the number of turns is wrong, but there is no arithmetic sequence with first term 60 and difference 6 whose sum is 864.
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alexb
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Jul-30-02, 08:06 PM (EST) |
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1. "RE: B. Banneker's Math Puzzles"
In response to message #0
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>The work that follows is equivalent to the proportion 4/24 = >48/x solved to find x = 864. Well, I do not see how x = 864 solves 4/24 = 48/x. It does not. The solution is x = 48×6 = 288. >1) Initially the dog is 30 dog leaps from the hare. That's correct. >2) The hare turns when the dog reaches the hare. Not necessarily. All it'says is >When the Dog was almost at her heels >She quickly turn’d, and down the fields The dog might have been close but not quite reaching the hare. >3) The hare gains 3 dog leap’s worth of yards at each turn. That's my understanding too. >4) The hare makes 6 turns. OK. >5) 4 times the number of hare leaps = 3 times the number of >dog leaps. No, it's the other way around: 3 times the number of hare leaps equals 4 times the number of dogs leaps. The dog made fewer leaps. Every time it made 3 leaps, the hare made four leaps. >6) 2 times the length of each dog leap = 3 times the length >of each hare leap. OK. >7) The question: How many leaps does the dog make in order >to catch the hare? Right. >Statements 5) and 6) combine to state that the dog runs >twice as fast as the hare. No, only 9/8 as fast. I think the problem is much simpler than any of your solution would suggest. At the outset, the distance between the dog and the hare was 30 dog leaps. With 6 turns the hare added 18 leaps more. We may assume that the problem is equivalent to both running on a straight line with dog starting 48 leaps away. |
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John Mahoney

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Jul-31-02, 09:46 PM (EST) |
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2. "RE: B. Banneker's Math Puzzles"
In response to message #1
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Ouch! I made a typing error. In copying Banneker's proportion, I wrote 24 instead of 72. The sentence I typed was: >>The work that follows is equivalent to the proportion 4/24 = >>48/x solved to find x = 864. It'should be: The work that follows is equivalent to the proportion 4/72 = 48/x solved to find x = 864. > >>2) The hare turns when the dog reaches the hare. > >Not necessarily. All it'says is > >>When the Dog was almost at her heels >>She quickly turn?d, and down the fields > >The dog might have been close but not quite reaching the >hare. I agree, but I'm unsure how to decipher the mathematical meaning behind these lines. I wonder, for example, about the need to make sure that the hare is not turning in the midst of a leap. >>5) 4 times the number of hare leaps = 3 times the number of >>dog leaps. > >No, it's the other way around: 3 times the number of hare >leaps equals 4 times the number of dogs leaps. The dog made >fewer leaps. Every time it made 3 leaps, the hare made four >leaps. Ouch again. I can't believe I made this error. You are quite correct. > >>Statements 5) and 6) combine to state that the dog runs >>twice as fast as the hare. > >No, only 9/8 as fast. I certainly agree now. >I think the problem is much simpler than any of your >solution would suggest. At the outset, the distance between >the dog and the hare was 30 dog leaps. With 6 turns the hare >added 18 leaps more. We may assume that the problem is >equivalent to both running on a straight line with dog >starting 48 leaps away I think I agree with you now that the problem is equivalent to the dog being 48 dog leaps away. In particular, it is now making sense that the turns, themselves, are not particularly important. If X is the number of leaps it takes the dog to catch the hare, and the dog is initially 48 leaps away from the hare, and the dog runs at 9/8 the speed of the hare, I get the proportion: (X-48)/X = 8/9 which yields X=432. That is, of course, half the desired answer. Two questions, then: First, where did I go wrong this time and how do I get 864 as the answer. Second, while it is great to see the number 48 in the proportion, Banneker also had the numbers 4 and 72 in his proportion 4/72=48/x. Also the problem suggests that the writer originally thought 72 (and then 144) were the solutions. Do you have any idea why Banneker's proportion is a valid way of solving the problem? I really appreciate your response and apologize for being particularly thick headed.
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John Mahoney

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Aug-01-02, 05:07 PM (EST) |
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4. "RE: B. Banneker's Math Puzzles"
In response to message #3
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Banneker's solution looks like the following - 4: 72 :: 48 48 576 288 4|3456 864 ans. There are horizontal lines above the 576, 3456, and 864, so it is clearly evident that he is just multiplying 72 by 48 to get 3456 and then dividing that product by 4 to get 864. That is why I said that his solution was equivalent to solving the proportion 4/72 = 48/X. I vaguely recall the double colon use in a proportion from older algebra texts. I continue to appreciate your help. John Mahoney |
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John Mahoney

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Aug-02-02, 04:25 PM (EST) |
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6. "RE: B. Banneker's Math Puzzles"
In response to message #5
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Thanks again for your continued interest in the Banneker puzzle. What I find particulary interesting are the lines in the rhyme about 72 being a likely solution. I don't see how one would arrive at that suppostion. Obviously 72 is the product of 8 and 9 the two numbers in the ratio of the speeds of the dog and hare, but so what. The rhyme then goes on to imply that since 72 doesn't work, one should consider multiples of 72. Both the rhyme and Banneker's solution obviously imply that 864 (12*72) is correct, but I still don't see how it meets the conditions of the problem. |
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John Mahoney

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Aug-09-02, 02:42 PM (EST) |
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7. "RE: B. Banneker's Math Puzzles"
In response to message #6
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Banneker, I believe, is using an old technique called False Position to solve this equation. It is also called single position (as there is a double position method). It is also called false assumption. James R. Newman refers to this method in his "World of Mathematics", 1956, p. 173 vol. 1. W. W. Rouse Ball refers to it in his "Short account of the history of Math", Dover, 1960, p. 209. It essentially is a method by which one starts with a guess - here Banneker seems to have started with 72. Then "perform the same operations with it as are described to be performed in the question." "Then say, as the sum of the errors is to the given sum, so is the supposed number to the true one required." So what this means is that somehow Banneker changed the problem into an equation, equal, I believe to 48 (which is the number of dog leaps the hare is ahead of the dog in Alex's reformulation of the problem.) By using the number 72, Banneker somehow got an answer of 4 (instead of 48) and therefore he formed the proportion 4/72 = 48/X. Of course I'm not sure that Banneker's 48 is the same as Alex's. And I certainly have no idea how Banneker "plugged" 72 into something and got 4. Any ideas? I'm still tearing what little hair I have out. Another of Banneker's puzzles is similar to the type of problem normally used to illustrate "double false position". In the biography of Banneker there is a reference by him that "he had advanced in (his self study of algebra) as far as Double Position". This gives credence to Banneker's knowledge of this method of problem solving. Briefly, double false position involves making two guesses for an equation or process whose desired answer should be zero. The method involves finding out where the line connecting the two sets of ordered pairs crosses the x- axis. If x1 and x2 are the guesses and f is the function, the method uses this quotient to find one a lot closer to the answer: X= (x2*f(x1)-x1*f(x2))/(f(x1)-f(x2)) Interestingly, this method doesn't require an equation for f. All it requires is two trials and their result! John Mahoney
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