Katie has a 64 ounce bottle of 91% alcohol. She needs to add some purified water and dilute the solution to 71% alcohol. How much water should she add?

18.02

Step-by-step explanation:

We have the following formula of dilutions in chemistry.

C1 * M1 = C2 * M2

We have a 64 ounce bottle of 91% alcohol. Therefore C1 91% and M1 64, and to decrease to 71% that is C2.

solving for M2

M2 = C1 * M1 / C2

Replacing

M2 = 91% * 64/71%

M2 = 82.02

M2 is the total, so you must add

82.02 - 64 = 18.02