Recall that the two acute angles of a right triangle add up to 90 degrees.
Let s be the measure (in degrees) of the smallest angle of the given right triangle, then we can set the following equation:

Adding like terms we get:

Subtracting 40 from the above result we get:

Dividing the above result by 2 we get:

Therefore the measure of the smallest angle is 25º.
Answer: 25º.