Task: Question Answering
Input (English): Problem: at a certain restaurant , the ratio of the number of cooks to the number of waiters is 3 to 10 . when 12 more waiters are hired , the ratio of the number of cooks to the number of waiters changes to 3 to 13 . how many cooks does the restaurant have ? 
Options: a ) 4 , b ) 6 , c ) 9 , d ) 12 , e ) 15
Output (English): d