Reasoning solution:
40 oz of 10% glycol contains 40*0.1=4 oz of glycol, and 40-4=36 oz. of water.
To make a 25% glycol using 36 oz of water will make a total volume of 36*(4/3)=48 oz.
Volume of pure glycol to be added = 48-40=8 oz
Therefore 8 oz of pure glycol must be added to the mixture to make a 25% solution.
Algebraic solution:
Let x = volume of pure glycol required in oz.
then
0.10*40+1.00*x = 0.25*(40+x)
expand
4.0+x=10+0.25x
solve
(1-0.25x)=10-4=6
x=6/0.75=8 oz
Therefore 8 oz of pure glycol must be added to the mixture to make a 25% solution.
