10 cubic centimeters of a 60% alcohol solution, would mean 10 x 60% = 6 cc is alcohol the other 4 cc is water.
To make a 90% solution, the water would be 10% of the solution.
If 4 cc is 10%, the total is 40 cc.
40 - 10 = 30 cc
30 cubic centimeters needs to be added.
