To find the length of a diagonal in a rectangle you use the Pythagoras theoren to find the length of the hypotenuse (x) in the right triangle formed by the diagonal length and height of the rectangle:
Pythagoras theorem:
[tex]\begin{gathered} h^2=l^2+l^2 \\ h=\sqrt[]{l^2+l^2} \end{gathered}[/tex]h is the hypotenuse
l are the legs of the right triangle
[tex]\begin{gathered} x=\sqrt[]{(20in)^2+(15in)^2} \\ \\ x=\sqrt[]{400in^2+225in^2} \\ \\ x=\sqrt[]{625in^2} \\ \\ x=25in \end{gathered}[/tex]Then the length of the diagonal is 25 inches
