we have to assume that pure base is being added to the alcohol

concentration = alcohol/total-volume

alcohol = concentration * total-volume

total-volume = alcohol/concentration

let 'x' represent the amount of 91% alcohol used

in 10 ounces of a 50% solution, we have this much pure alcohol:

0.5 * 10 = 5 ounces

we can obtain 5 ounces of pure alcohol from this much 91% alcohol:

5/0.91 = 5.49

we need to use approximately 5.49 ounces of 91% alcohol