It takes Avery 3 hours to build a brick wall while Tom can do it in 3 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?	Answers:
A) 20
B) 60
C) 30
D) 40
E) 50

Rationale: "avery takes 3 hours tom takes 3 hours efficiency of avery is 1 / 3 units / hr efficiency of tom is 1 / 3 units / hr combined efficiency of tom and avery is 1 / 3 + 1 / 3 = 2 / 3 units / hr since they worked for 1 hour they completed 2 / 3 units of work and 1 / 3 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 / 3 / 1 / 3 hours = > 1 * 60 = 60 minutes . . . answer will be ( b )"
Answer: B