Hermina cut a 10" by 15" piece of cardboard down the diagonal

When you cut along the diagonal, a right triangle is formed where the hypotenuse is the diagonal and the legs are the length and width of the rectangle (10 and 15 in this case)

Now use the pythagorean theorem to solve

10^2+15^2=325=c^2

c = approx 18.02775=length of the diagonal