Question 213862
A whole 3 sided triangle is equal to 180 so the two angels 15 and 85 are added with the third angel (x) and equals to 180 degrees.

15 + 85 + x = 180

Then you add the two numbers:

100 + x = 180

Subtract 100 from both sides:

100 + x = 180
-100      -100
_______________    
 x = 80         <--- And you should get 80 for the third angel! Simple is that!