In a triangle, one angle is 90 degrees and another is 35 degrees. What is the third angle?
Hints
Take them one at a time. The first gives nothing away.
1A nudge
The three angles add to 180 degrees.
2The strategy
Add the two angles you know, then subtract from the angle sum of a triangle. Nothing about the picture matters here beyond those two numbers.
3The full solution
90 + 35 = 125, and 180 - 125 = 55 degrees.
Solution
The reliable way
The angles of any triangle add to 180 degrees. Two of them are 90 and 35, which come to 125. The third is 180 - 125 = 55 degrees. The transferable idea: in a right-angled triangle the two non-right angles always add to 90, which is a faster route to the same answer.
The elegant way
In a right-angled triangle the other two angles must add to 90, so the third is simply 90 - 35 = 55.
Why this is on the test: Angle chases start here, and assuming a right-angled triangle is also isosceles is the first wrong turn students take.