A is 1.5 times as fast as B. A alone can do the work in 30 days. If A and B working together in how many days will the work be completed?  Choose the correct choice from the following.
A) 23
B) 18
C) 21
D) 24
E) 25

Rationale: "a can finish 1 work in 30 days b can finish 1 / 1.5 work in 30 days - since a is 1.5 faster than b this means b can finish 1 work in 30 * 1.5 days = 45 days now using the awesome gmat formula when two machines work together they can finish the job in = ab / ( a + b ) = 45 * 30 / ( 45 + 30 ) = 20 * 30 / 50 = 18 days so answer is b"
Answer: B