X can do a piece of work in 20days. He works at it for 10days and then Y finished it in 12days. How long will Y take to complete the work?
Choose the correct choice from the following choices:
A) 10  B) 12  C) 24  D) 29  E) 30

Rationale: "work done by x in 10 days = 10 * 1 / 20 = 1 / 2 remaining work = 1 - 1 / 2 = 1 / 2 1 / 2 work is done by y in 12 days whole work will be done by y in 12 * 2 = 24 days answer is c"
Answer: C