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The salinity of the Atlantic Ocean averages 37 parts per [#permalink]
10 Oct 2009, 18:17

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Question Stats:

75% (02:55) correct
25% (02:42) wrong based on 5 sessions

I'm having a h*ll of a time w/this one (POSSIBLE SPOILER ALERT:Scroll down to see my set-up. Perhaps, someone can tell me where I screwed up?):

The salinity of the Atlantic Ocean averages 37 parts per thousand. If 64 oz of water is collected and placed in the sun, how many oz of pure water would need to evaporate to raise the salinity to 45 parts per thousand?

Re: Salinity (Help!) [#permalink]
10 Oct 2009, 19:44

rvthryet wrote:

Bunuel wrote:

Well don't know how you did it, but here is my way:

Salinity=3.7% Water=64 Salt in it=64*3.7%=2.368

We need 2.368 to be 4.5% --> x*4.5%=2.368 --> x=52.62. So we should evaporate 64-52.62=11.37

This is the exact way that i followed.. But is there a standard formula that i can use for such questions?

You can use formula but the formula changes as the question changes. You can have formula for every problem in gmat but thats not possible to remember. So its better to understand the problem and use the logic to solve it.

Trying to remember and use formula in gmat probably costs you too much because there are hundreds of formulas to be remembered. Try to remember as few formulas (that are must) as possible. _________________

Re: Salinity (Help!) [#permalink]
12 Oct 2009, 12:31

I use a mixure formula: (% difference between the weaker solution and the desired solution) x (amount of weaker solution) = (% difference between the stronger solution and the desired solution) x (amount of stronger solution).

In this case: .045x = .008(64) x=11.37

I'm not recommending or endorsing formula use, but since you asked for it, it works well in this instance.

Re: Salinity (Help!) [#permalink]
13 Jan 2012, 06:13

think of the problem as though we have a mixture of 4.5% salinity and we need to add some water (0%) to make a final solution having 3.7% salinity. this would be reverse of what is stated in the question, but is easier to solve.

final ratio would be (weighted average) = 4.5-3.7 : 3.7-0 = 8:37 water needed = \frac{8}{45}*64 = 11.377 oz _________________

Re: The salinity of the Atlantic Ocean averages 37 parts per [#permalink]
18 Apr 2014, 23:31

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