If you take a paper that is 20 inches long and fold it across a diagonal that is 25 inches long, what must the width be?

Use Pythagorean theorem

   the diagonal is the hypotenuse    one leg is 20       find the other leg (width)

         h^2 = l₁ ²  + l₂ ²

         25²  = 20²  +  w^{2       Solve for w}