Solution - Simplification or other simple results

Step  1  :

             1
 Simplify   ——
            20

Equation at the end of step  1  :

   1    1
  —— • ——
  21   20

Step  2  :

             1
 Simplify   ——
            21

Equation at the end of step  2  :

   1    1
  —— • ——
  21   20

Step  3  :

Final result :

   1            
  ——— = 0.00238 
  420