Task: Question Answering
Input (English): Problem: on a certain farm the ratio of horses to cows is 4 : 1 . if the farm were to sell 15 horses and buy 15 cows , the ratio of horses to cows would then be 13 : 7 . after the transaction , how many more horses than cows would the farm own ? 
Options: a ) 30 , b ) 60 , c ) 75 , d ) 90 , e ) 105
Output (English): a