Use Pythagorean Theorem, a2 + b2 = c2
We have a and c
We need to find b
A = 15
Solve equation for b
b2 = c2 - a2 = 252 - 152 = 625 - 225 = 400
b = 20
The length of b is 20.
You can also recognize that this triangle resembles a 3,4,5 triangle scaled up, so this triangle would be 15,20,25 if you just scale up that triangle by 5.