On a certain farm the ratio of horses to cows is 5:1. If the farm were to sell 15 horses and buy 15 cows, the ratio of horses to cows would then be 17:7. After the transaction, how many more horses than cows would the farm own?	Select the correct option.
A) 30  B) 40  C) 50  D) 60  E) 70

Rationale: originally , there were 5 k horses and k cows . 7 ( 5 k - 15 ) = 17 ( k + 15 ) 35 k - 17 k = 255 + 105 18 k = 360 k = 20 the difference between horses and cows is ( 5 k - 15 ) - ( k + 15 ) = 4 k - 30 = 50 the answer is c .
Answer: C