For the purpose we will use solution dilution equation:
c1V1=c2V2
Where, c1 - concentration of stock solution; V1 - volume of stock solution needed to make the new solution; c2 - final concentration of new solution; V2 - final volume of new solution.
c1 = 100%
c2 = 75%
V1 = 1 gal
V2 = ?
When we plug values into the equation, we get following:
100 x 1 = 75 x V2
V2= 100/75 = 1.33 gal
Now we can determine the necessary volume of water:
V(water)= V2 - V1 = 1.33 - 1 = 0.33 gal water should be added
