A can complete a project in 20 days and B can complete the same project in 30 days. If A and B start working on the project together and A quits 15 days before the project is completed, in how many days total will the project be completed?	Choose the correct option:	A) 18	B) 21	C) 24	D) 27	E) 30

Rationale: a ' s rate is 1 / 20 of the project per day . b ' s rate is 1 / 30 of the project per day . the combined rate is 1 / 12 of the project per day . in the last 15 days , b can do 1 / 2 of the project . thus a and b must complete 1 / 2 of the project , which takes 6 days . the total number of days is 6 + 15 = 21 . the answer is b .
Answer: B