Rabbits have 40 more legs than chickens, so the number of chicken feet is only half that of rabbits, so the number of chickens is twice that of rabbits with the same number of feet.

40÷4= 10

Therefore, there are 10 rabbits over half of the chickens.

Chicken has (100-10) ÷ (1+1/2) = 60.

There are 100-60=40 rabbits.