Solution for #15

Let c be the number of chickens, and r be the number of rabbits.

          r + c = 72
        4r + 2c = 200 

To solve the equations, we multiply the first by two, then subtract the second.

        2r + 2c = 144
             2r = 56
              r = 28
              c = 44 

So there are 44 chickens and 28 rabbits in the cage.