Subject: Pay phone problem
Date: Mon, 23 Feb 1998 22:41:49
From: shameq@usa.net

Hi, I've been given a problem that I'm having some trouble with. I'd really appreciate any help. Heres the question (it's called the Pay Phone Problem)

"A pay phone will take only 10p, 20p, 50p, and 1 coins"(It's British).

"A woman has plenty of 10p and 20p coins. She has no other coins. She can put the coins into the pay phone in any order.

To make a call costing 50p, she could put the coins in the order

20p, 20p, 10p
OR
10p, 20p, 20p
OR
10p, 10p, 20p, 10p

There are more ways of making 50p with only 10p and 20p coins" (I worked out that there was 8 ways)

  1. The woman is going to make a phone call costing any multiple of 10 (i.e. 10p, 20p, 30p, 40p, etc.). INVESTIGATE the number of different ways, she could put the 10p and 20p coins into the pay phone. A man also wants to use the pay phone. He has plenty of 10p and 50p coins. He has no other coins. He wishes to make a phone call costing any multiple of 10p.
  2. INVESTIGATE the number of different ways he has of entering the 10p and 50p coins into the phone.
  3. INVESTIGATE the more general case."

This is what I've got so far:

For no.1 I've worked out a general equation where the next number in the series can be found if the previous two are known. The equation is: f of n = (f of n - 1)+(f of n - 2) where f=the number of ways of making (in this case) "n"

For example, I found the 1st number in the series to be 1 (i.e. she can only enter 10p in one way) The second number in the series was found to be 2 (i.e. she can enter 20p using 10p, 10p OR 20p)

Using the general equation the third number in the series was found to be 3, i.e.

f of n = 2+1 = 3

I know the equation works but I have no proof for it.

What I've got for #2 is pretty similar, except the genral equation is:

f of n= (f of n - 1)+(f of n - 5) Before this equation could be used, the first five in the series had to be found out first. As with #1, I have no proof for this.

The part that I'm really having trouble with is #3. I've got no idea what to do.

Your help would really be appreciated in finding a proof for the equations in #1 and #2, and also in whatever way you can help in #3.

Thanks alot,

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