Answer:
40 mm
Explanation:
It's 40 mm because the Pythagorean Theorem explains a^2 + b^2 = c^2.
This means the unknown number plus 30^2 = 50^2
What you want to do is 50^2 - 30^2= 20^2; which would be 2500-900=1600.
You got 1600 but that isn't your final answer. You want to find the square root 1600. So 1600 square rooted is 40.
40 mm is your final answer.