A can finish a job in 20 days and B in 30 days. They work on alternate days, starting with A. In how many days will the job be finished?

Explanation

Two days (A then B) complete 1/20 + 1/30 = 1/12, so 11 such pairs (22 days) finish 11/12; A's 23rd day adds 1/20 and B's 24th day adds the last 1/30, so 24 days.