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500 sc test6#19

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by dunkin77 » Thu Apr 05, 2007 2:55 pm
Hi,

The answer is C) but cant seem to get the answer.... can you help??

How many liters of pure alcohol must be added to a 100-liter solution that is 20 percent alcohol in order to produce a solution that is 25 percent alcohol?
(A) 7/2
(B) 5
(C) 20/3
(D) 8
(E) 39/4
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by jayhawk2001 » Thu Apr 05, 2007 8:54 pm
Let x be the amount of alcohol that needs to be added

(20+x) / (100+x) = 25 / 100

Solving for x, we get 20/3
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by dunkin77 » Thu Apr 05, 2007 9:37 pm
I see - the point is that we need to add X to both original water and original alchol.

Thanks!
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by Cybermusings » Sat Apr 07, 2007 12:19 am
How many liters of pure alcohol must be added to a 100-liter solution that is 20 percent alcohol in order to produce a solution that is 25 percent alcohol?
(A) 7/2
(B) 5
(C) 20/3
(D) 8
(E) 39/4

100 liter solution which is 20% alchohol ; Thus it already has 20 liters of alchohol.

Suppose x litres must be added..

Now (20 + x / 100 + x)*100 = 25

2000 + 100x = 2500 + 25x

75x = 500

Thus x = 500/75 = 20/3 litres

Hope this helps!
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