ducks have two legs while dogs have four legs and their are 25 heads, so there must be 15 dogs and 10 ducks.
15(heads) x 4(legs/head) = 60 legs 10 (heads) x 2 (legs/head) = 20 legs
60 legs + 20 legs = 80 legs
15 heads + 10 heads = 25 heads
(legs/head) is read as legs per head