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 b quits 15 days before the project is completed , in how many days total will the project be completed ? 
Options: a ) 12 , b ) 14 , c ) 16 , d ) 18 , e ) 20
Output (English): d