a number is 20 less thank its square. find all numbers

Solution:

                Number = x

                  x = x^2 - 20

  x^2 - x - 20 = 0

x^2 - 5x + 4x -20 = 0

x(x - 5) + 4(x -5) = 0

(x-5)(x+4) = 0

 x = 5  or x = -4