7 cats and 3 dogs weigh as much as 4 monkeys. 1 monkey weighs as much as 1 cat and 1 dog. How many cats weigh as much as one dog?

This is an exercise in adding equations.  So c, d, and m for cat, dog and monkey leads us to:

1)  7c + 3d = 4m

2)  1c + 1d = 1m

Adding these 2 equations gives:

3)  8c + 4d = 5m

Eight cats plus four dogs weighs the same as 5 monkeys. But by eq. 2, a monkey weighs as much as a cat and dog, so we can eliminate monkeys by substituting eq. 2 for m in eq. 3:

4)  8c + 4d = 5(1c + 1d)

5)  8c + 4d = 5c + 5d

6)  3c = 1d

So 3 cats weigh as much as a dog! If you substitute this value into eq. 2, you'll see also that a monkey weighs 4 cats.