We can solve this problem using the Pythagorean Theorem. The Pythagorean Theorem states, that with hypotenuse c, and legs a and b, a² +b² = c², and since this is a right triangle, that principle is true here. In this problem, we have the length of the hypotenuse and leg a. So, we can insert the values of the variables and then simplify to find out the length of b.
15²+b²=20²
b²=400-225
b²=175
b=√175