16 mm
Explanation
Step 1
we have a rigth triangle so we can use the Pythagoras theorem
[tex]\begin{gathered} \text{leg}^{^2}_1+leg_2^2=potenuse^2 \\ \text{replacing} \\ 12^2+leg^2=20^2 \\ 144+Leg^2=400 \\ \text{subtract 144 in both sides} \\ 144+\text{leg}^2-144=400-144 \\ \text{leg}^2=256 \\ \text{leg}=\sqrt[]{256} \\ \text{leg}=16 \end{gathered}[/tex]I hope this help you
