Question 70908
Let x be amount of 20% peroxide solution
The amount of peroxide is .2x
100-x is the amount of 70% peroxide solution
This contains .7(100-x) peroxide

The amount of peroxide of these solutions combines to a total of

  .2x + .7(100-x) 

This forms a solution of 100 L which is 35% peroxide and thus contains
.35(100) peroxide

 So solve  .2x + .7(100-x) = .35(100) for x 

x is the amount of 20% peroxide solution
100 -x is the amount of 70% solution