Question

In what proportion should you combine 2:5 soaking solution and distilled water to create 1:20 solution?

Answer 1

Let's make things easy by ending up with 21L of the 1:20 solution. That will contain 1L of pure solution.

(2/7)x + 0(21-x) = (1/21)(21)
x=7/2

So, mix 3.5L of the 2:5 mixture with 17.5L of water