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 choice from the following choices.
A) 18	B) 21	C) 24	D) 27	E) 30
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= 
<gadget id="calculator"> 6 + 15</gadget>
<output>21</output>
21.   The answer is 21.
<result>B</result>
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?