A and B together can complete a piece of work in 4 days. If A alone can complete the same work in 12 days, in how many days can B alone complete that work?

Explanation

A + B complete 1/4 of the work per day. A alone completes 1/12. Therefore, B's rate is:

1/4 - 1/12 = 1/6

So B alone needs 6 days.