A is 1.5 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) 22
C) 12
D) 24
E) 25

Rationale: a can finish 1 work in 20 days b can finish 1 / 1.5 work in 20 days - since a is 1.5 faster than b this means b can finish 1 work in 20 * 1.5 days = 30 days now using the awesome gmat formula when two machines work together they can finish the job in = ab / ( a + b ) = 20 * 30 / ( 20 + 30 ) = 20 * 30 / 50 = 12 days so answer is c
Answer: C