Right Triangle Angle Calculation: Finding Angle C Given A = 20°

To solve this problem, we need to determine the measure of angle $∢C$ in the right triangle $\triangle ABC$ where angle $∢A = 20°$.

Since $\triangle ABC$ is a right triangle, we know that one angle, $∢B$, is $90°$. Hence, the other two angles, $∢A$ and $∢C$, must sum to $90°$ as well.

We are given that $∢A = 20°$. Therefore, we can set up the equation:$∢A + ∢C = 90°$

Substitute the given value of $∢A$ into the equation:
$20° + ∢C = 90°$

To solve for $∢C$, subtract $20°$ from both sides:
$∢C = 90° - 20°$

Thus, we find that:
$∢C = 70°$

Therefore, the size of angle $∢C$ is $\textbf{70°}$.