Given:
The length of the two sides of the right triangle is 20 units and 15 units.
Use the Pythagorean theorem to find the third side of the triangle,
[tex]\begin{gathered} \text{hypotenuse}^2=side^2+side^2 \\ =20^2+15^2 \\ =400+225 \\ =625 \\ \text{Take square root on both sides} \\ \text{hypotenuse}=\pm\sqrt[]{625} \\ =\pm\sqrt[]{25^2} \\ =\pm25 \\ \text{But length can not be negative} \\ \Rightarrow\text{hypotenuse}=25 \end{gathered}[/tex]Answer: The length of the third side is 25 units.