If the measure of an angle is twenty less than the measure of its complement, what is the measure of the angle

Assuming the measurement is in degrees:

The sum of an angle and its complement is 90°.
Assign the variable x to the angle. Since it is 20° less than its complement, the angle's complement can be written as x+20°. 

With this information, an equation can be written:
x+x+20°=90°
2x+20°=90°
2x=70°
x=35

The measure of the angle is 35°.

because x-20+x=90. 2x-20=90. 2x=110 x=55. Then you fill in for the angle. (55)-20=35. Angle=35. Now double check. 55-20+55=90 35+55=90. 90=90✔️