SOLUTION: how much pure alcohol must be added to 15 oz of a 60% solution of rubbing alcohol to change it to a 70% solution?

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Question 311236: how much pure alcohol must be added to 15 oz of a 60% solution of rubbing alcohol to change it to a 70% solution?
Found 2 solutions by unlockmath, mananth:
Answer by unlockmath(1688) About Me  (Show Source):
You can put this solution on YOUR website!
Hello,
Let's set this up as a proportion.
15/.6=n/.7 We can cross multiply to get:
10.5=.6n Divide both sides by .6 to get:
n=17.5
So we need to add 2.5 oz.
RJ

Answer by mananth(16946) About Me  (Show Source):
You can put this solution on YOUR website!
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