If a hen and a half lay an egg and a half in a day and a half, how long will it take ten hens to lay ten eggs?

Ah, this famous old chestnut.

hens | eggs | days | ||

1½ | 1½ | 1½ | ||

So | 3 | 3 | 1½ | Double the hens and we double the eggs. So each hen lays one egg every 1½ days. |

Therefore, | 10 | 10 | 1½ |

So the answer is 1½ days.