It takes Avery 4 hours to build a brick wall while Tom can do it in 2 hours. If the two start working together and after an hour Avery leaves, how much time will it take Tom to complete the wall on his own?  Choose the correct choice:
A) 20 B) 40 C) 30 D) 50 E) 60

Rationale: "avery takes 4 hours tom takes 2 hours efficiency of avery is 1 / 4 units / hr efficiency of tom is 1 / 2 units / hr combined efficiency of tom and avery is 1 / 4 + 1 / 2 = 3 / 4 units / hr since they worked for 1 hour they completed 3 / 4 units of work and 1 / 4 units of work is left which is to be completed by tom ( since avery left ) so time taken by tom to complete the remaining work will be 1 / 4 / 1 / 2 hours = > 1 / 2 * 60 = 30 minutes . . . answer will be ( c )"
Answer: C