Question

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

Answer 1

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 :-)

Answer 2

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.