Question

A mixture contains forty ounces of glycol and water and is ten percent glycol. If the mixture is to be strengthened to twenty-five percent, how much glycol is to be added?
4 oz
8 oz
10 oz

Answer 1

10(40) + x = .25(x + 40)
 
4 + x = .25x + 10
 
x - .25x = 10 - 4
 
.75x = 6
x =
x = 8 oz of glycol required

Answer 2

If we add 8 oz of glycol to the 40 oz mixture we have 48 oz of mixture and 4+8 = 12 oz glycol
 12 / 48  = 1/4   = 25 % glycol 

the answer is 8 oz