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?
Choose the correct choice from the following options:
A) 20 B) 30 C) 40 D) 50 E) 60
Originally, there were 3k horses and k cows.   3(3k-15) = 5(k+15)   9k-5k = 75+45   4k = 120   k = 30   The difference between horses and cows is (3k-15)-(k+15) = 2k-30 = 30   The answer is 30.
<result>B</result>
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?