Afunction is defined on the set of real numbers by f(x)= 4-x/x find in its simplest form an expression for f(f(x))

Replace the x in (4-x)/x  by (4-x)/x:-

=  4  - (4 - x)             (4 - x)
             /       
              x                   x

=  4x - 4 + x               x
        *       
           x                   (4 - x)

=       5x - 4
               Answer
          4 - x

The answer for this question is 80