Task: Question Answering
Input (English): Problem: a can do a work in 21 days and b alone can do it 28 days , they begin the work together , but a left after some days . b completed the remaining work in 21 days . after how many days did a leave ? 
Options: a ) 10 , b ) 8 , c ) 3 , d ) 15 , e ) 16
Output (English): c