If the measure of an angle equals twice the measure of its supplement what is the measure of the angle?

120

Consider: You know that the supplement of an angle is equal to 180 minus the angle. You also know that an angle is twice it's supplement.

Now, if you think of angle as being "A", and it's supplement being "S", then we can re-express the above statements like so:

S = 180 - A

A = 2S

Then logically, we can replace the "S" in the second equation with the value we have for it in the first one:

A = 2( 180 - A)

and then solve that equation:

∴ A = 360 - 2A

∴ A + 2A = 360

∴ 3A = 360

∴ A = 360/3

∴ A = 120