Answer:
14.3 - Not entirely sure?
Explanation:
So you first divide 260 by 3 (the riders), and if they used a total of 18 horses together you then divide them by 3 as well. You get the answer from 260 and 3 (which would be 86) also the answer from 18 and 3 (which would be 6) and divide them together (I know, lots of dividing) and there's your answer! 14.3