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 5 days before the project is completed, in how many days total will the project be completed?
Choose the correct answer
A) 12
B) 15
C) 18
D) 21
E) 24
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 5 days, B can do 1/6 of the project.   Thus A and B must complete 5/6 of the project, which takes 10 days.   The total number of days is  10 + 5= 
<gadget id="calculator"> 10 + 5</gadget>
<output>15</output>
15.   The answer is 15.
<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 5 days before the project is completed, in how many days total will the project be completed?