In a right-angled triangle, one of the angles is 50 degrees, find the degree measure of the other angle?

Given:
∆ABС;
∠С = 90 °;
∠А = 50 °;
∠В =?
Decision:

Consider ∆ABС. By the condition of the problem, it is rectangular.
In a right-angled triangle, the sum of two acute angles is 90 °.
Then:
∠А + ∠В = 90 °;

From here we find what the angle B is equal to.
∠В = 90 ° – ∠А;
∠В = 90 ° – 50 ° = 40 °;
Answer: ∠В = 40 °.