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How many liters of pure alcohol must be added to a 100 liter

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Manager
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How many liters of pure alcohol must be added to a 100 liter [#permalink] New post 26 Jul 2007, 18:47
How many liters of pure alcohol must be added to a 100 liter solution that is 20% alcohol in order to produce a solution that is 25% alcohol?

a)7/2
b)5
c)20/3
d)8
e)39/4

Please explain your approach.

Thanks,
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 [#permalink] New post 26 Jul 2007, 19:55
It has to be atleast above 5. So 1 and 2 ruled out

28/108 is too high and that would rule out the other option E too.

so by POE, it must be C.

26.66/106.66 sounds right too.
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 [#permalink] New post 27 Jul 2007, 01:16
100 litres -> 20% = alcohol = 20 litres alcohol

Add x litres. So now total = (100+x)litres.

25/100 * (100+x) = 20+x
25 + 0.25x = 20+x
5 = 0.75x
x = 20/3
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a slightly different approach [#permalink] New post 17 Nov 2007, 19:37
I did this in a slightly different way :

|100-25| = 75
|20-25| = 5

5:75 reduces to 1:15

15/16 = 100/x

x= 106 2/3

106 2/3 - 100 = 6 2/3

6 2/3 = 20/3
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 [#permalink] New post 18 Nov 2007, 04:48
ywilfred wrote:
100 litres -> 20% = alcohol = 20 litres alcohol

Add x litres. So now total = (100+x)litres.

25/100 * (100+x) = 20+x
25 + 0.25x = 20+x
5 = 0.75x
x = 20/3


Solved it the same way to get C.
  [#permalink] 18 Nov 2007, 04:48
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How many liters of pure alcohol must be added to a 100 liter

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