Q: At a zoo, the number of two-legged birds is twice the number of 4-legged mammals.

If there is a total of 80 legs, how many birds and how many 4-legged mammals are there?

A: There are 20 two-legged birds (40 legs) and 10 four-legged mammals (40 more legs).
Let x = the number of 4-legged mammals
then 2x = # of 2-legged birds
2x(2) + x (4) = 80
8x = 80
x = 10 2x = 20