Task: Question Answering
Input (English): Problem: 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 ? 
Options: a ) 18 , b ) 21 , c ) 24 , d ) 27 , e ) 30
Output (English): b