A 400-milliliter solution is 20% alcohol by volume. If 100 milliliters of water is added, what is the new concentration

[Vincen]

A 400-milliliter solution is 20% alcohol by volume. If 100 milliliters of water is added, what is the new concentration of alcohol, as a percent of volume?

(A) 5%
(B) 10%
(C) 12%
(D) 12.5%
(E) 16%

Answer: E

Source: Manhattan GMAT

[Brent@GMATPrepNow]

A 400-milliliter solution is 20% alcohol by volume. If 100 milliliters of water is added, what is the new concentration of alcohol, as a percent of volume?

(A) 5%
(B) 10%
(C) 12%
(D) 12.5%
(E) 16%

Answer: E

Source: Manhattan GMAT

A 400 milliliter solution is 20% alcohol by volume.
20% of 400 = 80
So, the original solution contains 80 milliliters of alcohol

If 100 milliliters of water is added...
400 + 100 = 500
So, our NEW solution has a total volume of 500 milliliters

...what is the new concentration of alcohol, as a percent of volume?
NEW concentration of alcohol = 80/500
Divide top and bottom of our fraction by 5 to get: 16/100, which is the same as 16%

Answer: E

Cheers,
Brent