Let's call the length 4x and the width 3x. Then the perimeter is
[tex]p = 4x + 3x + 4x + 3x = 14x[/tex]
The diagonal is given by the Pythagorean Theorem
[tex]20^2 = (4x)^2 + (3x)^2 [/tex]
[tex]4^2 (5^2)= (4^2 + 3^2) x^2[/tex]
[tex]4^2 (5^2)= (5^2) x^2[/tex]
[tex] x^2 = 4^2[/tex]
We're only interested in positive lengths,
[tex]x = 4[/tex]
[tex]p = 14(4) = 56[/tex]
Answer: 56