A farmer counts 70 heads and 200 legs how many chickens and pigs does he have?

Let c represent the number of chickens and p represent the number of pigs.

Since chickens and pigs have one head each, 70 heads = c + p.

Since chickens have 2 legs and pigs have 4 legs, 200 legs = 2c + 4p.

Subtracting twice the first equation (140 = 2c + 2p) from the second, we get
60 = 2p
or
p = 30 pigs

Rewriting the first equation as c = 70 - p, we get c = 40 chickens.