The salinity of the Atlantic Ocean averages 37 parts per thousand. If

Question

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?

A. 10.42
B. 11.38
C. 12.56
D. 14.33
E. 15.44

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?  ):

(x)(45/1000) = (64)(37/1000) - (64-x)(37/1000)

Bunuel · Accepted Answer

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=11.38

rvthryet · Answer

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

GMAT TIGER · Answer

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.

ggarr · Answer

thanks. your solution is much simpler.

mendelay · Answer

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.

Posted from my mobile device

gmatjon · Answer

I found the explanation of bunuel much easier than memorizing any standard formula for this type of questions.
Thank you

ctrlaltdel · Answer

Economist method:(https://gmatclub.com:8080/forum/viewtopic.php?p=618107)

h2polo · Answer

Getting very familiar with this table is the key to success regarding mixture problems.

Mixture problems used to be very difficult for me, until I started using these tables to organize the facts.

I do agree with pierrealexandre77 that this is not representative of what you would see in a GMAT problem...

Bullet · Answer

HI ctrlaltdel

If you can please explain from where you got the values in the table like 0.963 and the other.

I also use the table but sometimes it didnt serve the purpose mentioned in the question so i need to use alternate methods.

If you can please explain then it'd be great.

Thanks

MBAhereIcome · Answer

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 = \(\frac{8}{45}*64 = 11.377 oz\)

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

Kinshook · Answer

Given: The salinity of the Atlantic Ocean averages 37 parts per thousand. 64 oz of water is collected and placed in the sun

Asked: How many oz of pure water would need to evaporate to raise the salinity to 45 parts per thousand?

In 1000 parts of saline water , salt part = 37
In 1000 parts of saline water , water part = 963

Let water to be evaporated to raise the salinity to 45 parts per thousand be x
37/1000-x = 45/1000
45000 - 45x = 37000
x = 8000/45 = 1600/9

In 64 oz of water , water to be evaporated = 1600/9 * 64/1000 = 16*64/90 = 11.38 oz

PinakiRDas · Answer

I took a more cumbersome approach: (37/1000X64)/(64-x)=45/1000 This is more time consuming

pappal · Answer

salinity=3.7% 
amount of salt=3.7% of 64 oz of ocean water taken
if we assume x oz of water gets evaporated to make the salinity 4.5%
hence
3.7% of 64 oz=4.5% of (64-x) oz i.e. we equate the amount of the salt in the two solutions, one before and the other after the evaporation, which will be a constant value.
=>37*64=45*64 - 45x
45x=8*64
x=11.38 oz

The salinity of the Atlantic Ocean averages 37 parts per thousand. If

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The salinity of the Atlantic Ocean averages 37 parts per thousand. If

Prep Toolkit

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