Question

Two angles of a triangle measure 15° and 85 °. what is the measure for the third angle?

Answer 1

the sum of the three angles of a triangle (in Euclidean Geometry) will always be 180 degrees..

so let x degrees be the third angle.

We know 15 + 85 + x = 180
this means -> x = 180 - 15 - 85 = 80
So your third angle is 80 degrees.

Answer 2

All three angles of a triangle must equal 180* so use this formula then solve.
15+85+x=180
90+x=180
-90 -90
x=90

Your last angle is 90*

Hope this helps :-)