An angle's measure is twice its supplement's what is the measure of the two angles?

A supplemental angle "completes" a given angle to make it a straight angle (180 degrees). The supplement of 90 is 90. The supplement of 100 is 80. As long as the two angles add up to 180 they supplement each other.

So, you have some angle, x and its supplement, 180-x.

The supplement is twice as big as the angle.

180-x = 2*x

Solve for x and you're done!