This is a specially easy version of what is usually a harder problem. Almost too easy. Are you sure the numbers are right?

In general you would solve two simultaneous linear equations, one for the heads and one for the feet.

(Heads) C + P = H [here 50]
(Feet) 2C + 4P = F [here 100]

Another way is to first see how many feet you would have if you had only chickens. Of course, if the answer was 100 you'd be done.

But suppose it isn't. Then substituting a pig for a chicken doesn't change the number of heads and adds two feet. Figure out how many times you'd need to do this and you're done.

Good hunting, er, farming!
RD