Question 311236
let quantity of 100% alcohol required be x oz

rubbing alcohol = 60% 15 oz

Mixture 70% alcohol quantity = (x+15) oz

add the concentrations of individual and equate it to total concentration
.
1x+0.6*15 =0.7(x+15)

x+9=0.7x +10.5

x-0.7x = 10.5-9
0.3x = 1.5
x=5 oz of pure alcohol