Use pythagorean theorem to solve the problem. The sum of the squares of the perpendicular legs is equal to the square of the hypotenuse. If the length of the other leg is represented by x, then the equation will be:
x² + 4² = 15²
x² + 16 = 225
x² = 225 - 16
x² = 209
x = √209
The length of the other leg is √209 units