Legs in the Barn

4 pigs

One way to do this problem is to use a table. Before we start though, notice: (1)that if one third of the animals are chickens then two thirds are pigs and (2)since there are a whole number of chickens there must be three times a whole number of animals.

animals	chickens	pigs	legs
3	1	2	10
6	2	4	20