If a hen and a half lays an egg and a half in a day and a half, how long will it take ten hens to lay ten eggs? (This is a famous old problem.)
Solution
Given: hens 1½, days 1½, eggs 1½.
Doubling the hens but not the time: hens 3, days 1½, eggs 3.
Therefore: hens 10, days 1½, eggs 10.
Each hen lays one egg every 1½ days and the answer is 1½ days.