I think you mean to say "hypotenuse". If so, then use the pythagorean theorem to get the following
a^2 + b^2 = c^2
16^2 + 20^2 = c^2
256 + 400 = c^2
656 = c^2
c^2 = 656
c = sqrt(656)
c = sqrt(16*41)
c = sqrt(16)*sqrt(41)
c = 4*sqrt(41)
The hypotenuse is exactly 4*sqrt(41) units long
This is approximately 25.612 when you use your calculator.