The answer is 3/2.
Explanation: you want to get x by itself so that you have the equation x = 15. To do that you multiply 2/3x * 3/2 to get x. this works because 3/2 cancels out 2/3 and you get 1. Multiplying 10 by 3/2 also gets you 15. So in the end you have
x = 15.
