Task: Question Answering
Input (English): Problem: when a person aged 39 is added to a group of n people , the average age increases by 2 . when a person aged 15 is added instead , the average age decreases by 1 . what is the value of r ? 
Options: a ) 7 , b ) 8 , c ) 9 , d ) 10 , e ) 11
Output (English): a