14:
7/10 equals 21/30 so he would need to throw the ball 21 times.
15:
A, D and F
(Explanation: 7 times 8 is 56 so they all would inflate 7 balloons in option A. 8 times 8 is 64 so they all would inflate 8 balloons in option D. 12 times 8 is 96 so they all would inflate 12 balloons in option F.)