A and B can together finish a work in 40days. They worked together for 10days and then B left. After another 9days, A finished the remaining work. In how many days A alone can finish the job?  Choose one
A) 12 B) 25 C) 60 D) 30 E) 20

Rationale: "a + b 10 days work = 10 * 1 / 40 = 1 / 4 remaining work = 1 - 1 / 4 = 3 / 4 3 / 4 work is done by a in 9 days whole work will be done by a in 9 * 4 / 3 = 12 days answer is a"
Answer: A