Task: Question Answering
Input (English): Problem: tony alone can paint a wall in 7 days and his friend roy alone can paint the same wall in 9 days . in how many days they can paint the wall working together ? round off the answer to the nearest integer . 
Options: a ) 2 , b ) 4 , c ) 6 , d ) 7 , e ) 9
Output (English): b