In a farm there are cows and hens. If heads are counted there are 180, if legs are counted there are 420. The number of cows in the farm is :

Answer: Option D

Let the number of cows be x.
Then, number of hens would be (180 - x)
According to the question
(180 - x) × 2 + 4x = 420
⇒ 360 - 2x + 4x = 420
⇒ 2x = 420 - 360 = 60
⇒ 2x = 60
∴ x = $$\frac{60}{2}$$ = 30