Product of numbers close to 100
Say, you have to multiply 94 and 98. Take their differences to 100: 100  94 = 6 and 100  98 = 2. Note that 94  2 = 98  6 so that for the next step it is not important which one you use, but you'll need the result: 92. These will be the first two digits of the product. The last two are just 2×6 = 12. Therefore, 94×98 = 9212.
The same trick works with numbers above 100: 93×102 = 9486. First find the differences: 100  93 = 7 and 100  102 = 2. Then subtract one of the differences from the other number, e.g. 93  (2) = 95. This intends to show the first two digits of the product, i.e., the number 9500. Add to this the product of 7 and 2, or 14: 9500  14 = 9486.
Why does this work?
(100  a)(100  b)  = (100  a)·100  (100  a)·b 
 = (100  a)·100  100b + ab 
 = (100  a  b)·100 + ab. 
The method works for numbers larger than 100 and when the two straddle the 100 divide. In general

(100 + x)(100 + y)  = (100 + x)·100 + (100 + x)·y 
 = (100 + x)·100 + 100y + xy 
 = (100 + x + y)·100 + xy, 

where x and y are arbitrary (even real, not necessarily integer) numbers. If A = 100 + x and B = 100 + y, the formula says that
A×B  = (A + y)·100 + xy, or 
A×B  = (B + x)·100 + xy. 
For example, if A = 115 and B = 98 then x = 15 and y = 2 so that
115×98  = (115  2)·100 + 15×(2), or 
 = 11300  30 
 = 11270. 
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