When Farmer Fiona raises cows and chickens in all her animals have 20 heads and 70 legs?

Answer 1

Assuming that all of the animals are "normal" (no amputees, 2-headed mutations, etc.) :

15 cows (15 x 4 = 60 legs) plus 5 chickens (5 x 2 = 10 legs) gives a total of 20 heads and 70 legs.