Let x and y represent the two angles.
.. x + y = 90 . . . . . . two angles are complementary
.. x -2y = 15 . . . . . . one angle is 15 more than twice the other (y = smaller angle)
Subtract the second equation from the first.
.. (x +y) -(x -2y) = (90) -(15)
.. 3y = 75
.. y = 25
The smaller angle is 25 degrees.