Sum of Angles in a Triangle

While in college, your little brother sends you the following problem in the mail. It was on his last homework and he got it wrong, but he doesn't know why: \begin{align*} &\text{If you have a triangle where one interior angle is $70^\circ$ and another is $50^\circ$, what is the third angle?}\ &\text{Place your answer in the box.}\ &\boxed{\cancel{\color{blue}{90^\circ}}} \end{align*}

The crossed out answer was your brother's answer. What is the correct answer to this problem?