A is 2 times as fast as B. A alone can do the work in 20 days. If A and B working together in how many days will the work be completed? Answers.
A) 23  B) 13  C) 21  D) 24  E) 25

Rationale: "a can finish 1 work in 20 days b can finish 1 / 2 work in 20 days - since a is 2 faster than b this means b can finish 1 work in 20 * 2 days = 40 days now using the awesome gmat formula when two machines work together they can finish the job in = ab / ( a + b ) = 20 * 40 / ( 20 + 40 ) = 13 days so answer is b"
Answer: B