how many different coin combinations make 32 pence
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If we're talking about a monetary system with 1 pence, 2 pence, 5 pence, 10 pences, and 20 pence coins only, then:

The answer appears to be 128 ways.  Work shown below.

It's been a while since I've done one of these, so anyone can feel free to correct me if I made a mistake.

If you want to include half-pence, the number of ways will be larger.

 

If we look at just the 1 pence and 2 pences coins then there are 17 ways using from 0 to 16 of the 2 pence coins, with the rest being 1 pence coins.

If n is even, the # of ways to make a total of n pence using just 1 pence or 2 pence coins is (n+2)/2.

If n is odd, the # of ways would be (n+1)/2.

What are the ways we can use 5 pence, 10 pence, and 20 pence coins to make 32 pence?  (the balance will be made with 1 pence and 2 pence coins)

Table:  # of 20 pence, # of 10 pence, # of 5 pence, total, amount left to make 32

1x20, 1x10, 0x5, 30, 2

1x20, 0x10, 2x5, 30, 2

1x20, 0x10, 1x5, 25, 7

1x20, 0x10, 0x5, 20, 12

0x20, 3x10, 0x5, 30, 2

0x20, 2x10, 2x5, 30, 2

0x20, 2x10, 1x5, 25, 7

0x20, 2x10, 0x5, 20, 12

0x20, 1x10, 4x5, 30, 2

0x20, 1x10, 3x5, 25, 7

0x20, 1x10, 2x5, 20, 12

0x20, 1x10, 1x5, 15, 17

0x20, 1x10, 0x5, 10, 22

0x20, 0x10, 6x5, 30, 2

0x20, 0x10, 5x5, 25, 7

0x20, 0x10, 4x5, 20, 12

0x20, 0x10, 3x5, 15, 17

0x20, 0x10, 2x5, 10, 22

0x20, 0x10, 1x5, 5, 27

0x20, 0x10, 0x5, 0, 32

So that's 6 ways with 2 pence left, 4 ways with 7 pence left, 4 ways with 12 pence left, 2 ways with 17 pence left, 2 ways with 22 pence left, 1 way with 27 pence left, and 1 way with 32 pence left

For each of those amounts left (2, 7, 12, 17, 22, 27, 32) we can use (n+2)/2 and (n+1)/2 to figure out how many ways to make the remaining amounts with 1 pence and 2 pence coins

6 * (4/2) = 12

4 * (8/2) = 16

4 * (14/2) = 28

2 * (18/2) = 18

2 * (24/2) = 24

1 * (28/2) = 14

1 * (32/2) = 16

The total of 12 + 16 + 28 + 18 + 24 + 14 + 16 is 128.

 

 

by Level 13 User (103k points)

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