Question 730586
One would assume your concentrations are in %w/v, and you want how much water as m^3.


Let v = volume of water to add in units of cubic meters.
{{{highlight(500*40/(500+v)=25)}}}
Solve for v.


First couple of steps,
{{{500*40=25(500+v)}}}
{{{20*40=(500+v)}}}
{{{800-500=v}}}