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?
Choose the correct answer
A) 40 B) 50 C) 60 D) 70 E) 80

Rationale: "originally , there were 6 k horses and k cows . ( 6 k - 15 ) = 3 ( k + 15 ) 6 k - 3 k = 45 + 15 3 k = 60 k = 20 the difference between horses and cows is ( 6 k - 15 ) - ( k + 15 ) = 5 k - 30 = 70 the answer is d ."
Answer: D