Task: Question Answering
Input (English): Problem: a , b , c can do a piece of work in 20 days , 30 days and 40 days respectively , working alone . how soon can the work be done if a is assisted by b and c on alternate days ? 
Options: a ) 3 days , b ) 1 day , c ) 5 days , d ) 10 days , e ) 7 days
Output (English): a