In this task, you need to provide the correct option for a given problem from the provided options.
Q: Problem: on a certain farm the ratio of horses to cows is 6 : 1 . if the farm were to sell 15 horses and buy 15 cows , the ratio of horses to cows would then be 3 : 1 . after the transaction , how many more horses than cows would the farm own ?
Options: a ) 40 , b ) 50 , c ) 60 , d ) 70 , e ) 80
A:
d