You will be given a definition of a task first, then some input of the task.
Given a math problem with context and a question and 5 answer choices, the task is to provide the correct answer choice based on the problem. You must choose one of the given answer choices by letter: a, b, c, d, or e; anything else is invalid.

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:
a