Question 65465
Let the length of the field be x m
Then the width = (x - 20)m

The perimeter = 2(l + w)
              = 2(x + x - 20)
              = 2(2x - 20)

Given the perimeter = 408.
THus the equation is 2(2x - 20) = 408
  ==> 2x - 20 = 204  [dividing by 2 throughout]
  ==> x - 10 = 102 [dividing by 2 again]
==> x = 102 + 10  [adding 10 to both the sides]
==> x = 112

Thus the length of the field  = 112 m

The width = x - 20
          = 112 - 20
          = 92 m


Good Luck !!!