A parallelogram has an interior angle measuring 81°. Which MUST also be the measure of an interior angle of the parallelogram?

Hey there! :D

Think of a parallelogram as a figure with two triangles. A triangle's interior angles add up to 180 degrees. So,

180 x 2= 360

The corresponding angles will always be supplementary. That interior angle (81 degrees) will be corresponding and supplementary to another angle. 

Let x equal the missing measure of the angle. 

81+x=180

Subtract 81 on both sides. 

x=99

Another measure of an interior angle must be 99 degrees. 

I hope this helps!
~kaikers