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°.