Question 956164
Treat like a triangle, for example, I'll use the your example, do the Pythagorean Theorem to find the missing side. In this case, the hypotenuse.

{{{20^2 + 15^2 = c^2}}}
{{{400 + 225 = c^2}}}
{{{625 = c^2}}}
{{{ sqrt(625) = sqrt(c)}}}
{{{25 = c}}}

The length of the diagonal is 25 m.