A can complete a project in 20 days and B can complete the same project in 40 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?
Answers:
A) 18 days B) 20 days C) 26.67 days D) 16 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 / 40 ) . ( 1 / 20 ) * ( x - 10 ) + ( 1 / 40 ) * ( x ) = 1 ( x / 20 ) + ( x / 40 ) - ( 10 / 20 ) = 1 3 x / 40 = 3 / 2 x = 20 therefore , the answer is b : 20 ."
Answer: B