A zookeeper counted the heads of the animals in a zoo and found it to be 80. When he counted the legs of the animals he found it to be 260. If the zoo had either pigeons or horses, how many horses were there in the zoo?

This is a classic heads and legs problem. Let h = horses and p = pigeons. Then h + p = 80 (heads) and 4h + 2p = 260 (legs, since horses have 4 legs and pigeons have 2). Multiply first equation by 2: 2h + 2p = 160. Subtract from second equation: 2h = 100, so h = 50. Verification: 50 horses + 30 pigeons = 80 heads, and 50×4 + 30×2 = 200 + 60 = 260 legs. Options A (40), B (30), and D (60) don't satisfy both conditions.