Answer: Hello mate!
As I understand, a = 16 is a cathetus and c = 20 is the hypotenuse, and we want to find the value of b, the other cathetus.
By the Pythagorean theorem, we know that the square of the hypotenuse is equal to the sum of the squares of the cathetus.
This means that:
a^2 + b^2 = c^2
then, we have:
16^2 + b^2 = 20^2
b^2 = 20^2 - 16^2
b = √(20^2 - 16^2) = 12
so the triangle has cathetus a = 16 and b = 12, and a hipotenuse of lenght c = 20.