## 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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