hello from mrbilldoesmath!
in the attachment, l is the longest side of the right triangle and "c" is its hypotenuse. the pythagorean theorem gives
20^2 + l^2 = c^2 => as 20^2 = 400 and c = l+5
400+ l^2 = (l+5)^2 => (l+5)^2 = l^2 + 10l + 25
400 + l^2 = l^2 + 10l + 24 => subtract l^2 from both sides
400 = 10l + 25 => subtract 25 from both sides
400 -25 = 10l =>
375 = 10l =>
l = 375/1- = 37.5
hence c = l + 5 = 37.5 +5 = 42.5
an imaginary line would need to be drawn at any angle to the center of the side opposite the angle to reflect across an axis of symmetry so that a regular pentagon can carry onto itself. hope this answer .