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Challenge 9 Answer


Ducks and dogs

The total number of ducks and dogs is 21:

P + C = 21

Feet total is 54. Ducks have 2 paws and dogs have 4 paws. So:

2P + 4C = 54

So we have two equations. Isolating P at first we have:

P = 21-C

Substituting in the second equation we have:

2 (21C) + 4C = 54
42-2C + 4C = 54
2C = 54-42
2C = 12
C = 6

Now just find the P:

P = 21-C
P = 21-6
P = 15

There are 15 ducks and 6 dogs, so the difference is 15-6 = 9.

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