Bobby can build a house in 24 days. Danny can build the same house in 36 days. Andy can build the same house in 27 days. If Bobby and Danny work together for 8 days, and will be continued by Andy, how long does Andy take to complete the remaining work?
1/24 + 1/36 ==5/72
Bobby and Danny working together can finish the job in: 72 / 5 ==14.4 days.
8 / 14.4 =5/9 of the work has already been finished.
1 - 5/9 ==4/9 of the work remains to be finished by Andy. Since Andy can finish the job, working alone, in 27 days, therefore he can finish:
4 / 9 (of the remaining work) x 27 days ==108 / 9 ==12 days.