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 will the project be completed?  Pick: A) 18 days B) 27 days C) 26.67 days D) 15 days E) 12 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 - 5 ) + ( 1 / 30 ) * ( x ) = 1 ( x / 20 ) + ( x / 30 ) - ( 5 / 20 ) = 1 5 x / 60 = 5 / 4 x = 60 / 4 x = 15 therefore , the answer is d : 15 ."
Answer: D