A does 60% of the work in 18 days. He then calls in B, and they together finish the remaining work in 4 days. How long would B alone take to do the whole work?

A completes 60% in 18 days, so A's rate = 60/18 = 10/3 % per day. In 4 days with B, they complete 40%. A does 4×(10/3) = 40/3 %. B does 40 - 40/3 = 80/3 % in 4 days. B's rate = 20/3 % per day. Time for B alone = 100/(20/3) = 15 days.