We have to apply the pythagorean theorem, since it's a right triangle:
c^2 = a^2+ b^2
c is the hypotenuse of the triangle ( longest side = 25 in) and a and b are the other 2 legs.
Replacing:
25^2 = 15^2+b^2
Solve for b ( missing side)
625= 225 + b^2
625-225 = b^2
400 = b^2
√400 = b
20= b
20 inches