A and B can together finish a work in 30days. They worked together for 20days and then B left. After another 20days, A finished the remaining work. In how many days A alone can finish the job?
Choose one:  A) 40  B) 50  C) 54  D) 60  E) none of these

Rationale: "( a + b ) ' s 20 days work = ( 1 / 30 * 20 ) = 2 / 3 . remaining work = ( 1 - 2 / 3 ) = 1 / 3 . now , 1 / 3 work is done by a in 20 days . whole work will be done by a in ( 20 * 3 ) = 60 days . correct option : d"
Answer: D