A can complete a project in 20 days and B can complete the same project in 20 days. If A and B start working on the project together and A quits 10 days before the project is completed, in how many days will the project be completed?	Pick one.
A) 18 days B) 27 days C) 26.67 days D) 16 days E) 15 days

Rationale: "let x = the number of days taken to complete the project . the amount of work done by a is ( x - 10 ) * ( 1 / 20 ) . the amount of work done by b is ( x ) * ( 1 / 30 ) . ( 1 / 20 ) * ( x - 10 ) + ( 1 / 20 ) * ( x ) = 1 ( x / 20 ) + ( x / 20 ) - ( 10 / 20 ) = 1 x / 10 = 3 / 2 x = 15 therefore , the answer is e : 15 ."
Answer: E