Task: Question Answering
Input (English): Problem: on a certain farm the ratio of horses to cows is 3 : 1 . if the farm were to sell 15 horses and buy 15 cows , the ratio of horses to cows would then be 5 : 3 . after the transaction , how many more horses than cows would the farm own ? 
Options: a ) 20 , b ) 30 , c ) 40 , d ) 50 , e ) 60
Output (English): b