Chlorination is used in industry for the purification of water; 1gm so

Question

Chlorination is used in industry for the purification of water; 1gm solution of chlorine will remove half of the germs in 1 liter of water. If water containing less than 0.09% of germs is considered clean, what is the minimum amount of chlorine required to purify 150 liter of water containing 0.3 liter of harmful substances?

A. 1
B. 2
C. 3
D. 4
E. 5

A. 1
B. 2
C. 3
D. 4
E. 5

EMPOWERgmatRichC · Accepted Answer

Hi gauriranjekar,

manpreetsingh86 is correct - the highlighted number must be a typo....

Assuming that it's SUPPOSED to state "0.3" liters of harmful substances, here's how you can answer this question.

First, we have to figure out what .09% of 150 liters is...

1% = .01
.1% = .001
.09% = .0009

So, .0009(150) = .135 liters

We need enough doses of chlorine to get the 0.3 liters down to LESS than .135 liters. Each "dose" of chlorine will remove HALF of the harmful substances...

Start = 0.3 liters
After 1st dose = .15 liters
After 2nd dose = .075 liters

We're now below the .135 liters that we need to be less than.

Final Answer:  B

GMAT assassins aren't born, they're made,
Rich

manpreetsingh86 · Answer

hi, please check the highlighted portion. if it is correct. then we don't need to add any chlorine into the water why ?? 0.09% of 150 =0.135. i.e. the concentration of harmful substances ( germs) must be less than this amount. but the amount of harmful substances present in the water is already less than this amount. hence we don't need to add any chlorine into the water.

Gau0809 · Answer

Hi...thanks for highlighting.
It was a typo. It is 0.3 and not 0.03!!
I corrected it..

KarishmaB · Answer

Are you sure the question you typed is correct? Can you put up a screenshot from the original source? I think the question is ambiguous and options make no sense.

1 gm solution of chlorine removes half the germs in 1 liter of water. 150 liter of water will need 150 gm solution of chlorine to kill half the germs. Currently, the water has .3/150 *100 = .2% harmful substances.
Adding 150 gms of chlorine will make it .1% harmful substances.
Now the issue is that do we need another 150 gms of chlorine solution (which will bring the harmful substances down to .05%) or a small amount which will bring the harmful substance concentration down to 0.09% since we need to find the minimum amount of chlorine solution.

The options 1, 2, 3, 4, 5 are of no help!

Gau0809 · Answer

Hello

Yes, the question is right. I am attaching the screenshot anyway.
And as per the explanation provided in the question source, that additional one gram is required to bring down the harmful substances to 0.05%.
Hence the answer is 2 grams.

Gau0809 · Answer

Similar problem one-filter-decreases-the-concentration-of-harmful-substances-in-water-190750.html#p1464227

Bunuel · Answer

Similar questions to practice: if-2-3-of-the-air-in-a-tank-is-removed-with-each-stroke-of-a-128432.html if-3-4-of-the-mineral-deposits-in-a-reservoir-of-water-are-97300.html if-1-2-of-the-air-in-a-tank-is-removed-with-each-stroke-of-a-114943.html one-filter-decreases-the-concentration-of-harmful-substances-in-water-190750.html on-january-1-2076-lake-loser-contains-x-liters-of-water-b-82667.html

beattheheat · Answer

Is something wrong with the question?
If 1gm solution of chlorine will remove half of the germs in 1 liter of water, then 150 gms of chlorine will be required  twice  to purify 150 liter of water containing 0.3 liter of harmful substances.

eddyki · Answer

Maybe we need to point out  what is the minimum amount of chlorine in the solution . That would be 2 gm per liter.

KarishmaB · Answer

Right, but the quantity of water given is 150 liters, not 1 liter. So 150 gms of chlorine solution will bring the harmful substances down to 50% of the original amount. Also, the options do not specify the units of measurement - 1 gm etc. 
Something certainly wrong with this question.

rohit8865 · Answer

here 1gm of chlorine is removing 50% germs.
So 0.3lts germs in 150 lts water comprises 0.2% germs
so to bring quantity of germs below .09% to be considered good solution we need 1gm chlorine
this 1 gm chlorine remove 50% of .2% germs
means still we have 0.1%germs for which we need another 1 gm of chlorine which again removes 50% of 0.1% germs to 0.05%/
which is obviously <0.09%germs
So total 2gms needed.
Ans B

ShashankDave · Answer

I agree with you. I think the question must be worded to state that - "1 gram of chlorine cleans half of the impurities from every 1 litre of water". If the question is this, then, the answer would be B.

mj12345 · Answer

This is a poorly worded question. Can we rephrase the "1gm Chlorine is required to remove half the germs in 1 liter of water"? Or change the answer choices...

anje29 · Answer

I am amazed the source is GMAT Prep

hellosanthosh2k2 · Answer

150 litres of water contains 0.3 litres of harmful substances ( i am assuming harmful as germs)
so 1 litre of water contains 0.3/150 litres of germs.

(1/2)^n * 0.3/150 < 0.09/100, where n number of grams of chlorine required.
=> (1/2)^n < 0.45
=> we need minimum of 2 gms of chlorine

altairahmad · Answer

This question has attained the 700 level rank because it is so poorly worded.

warriortx · Answer

1. Calculate concentration in sample:

0.3L / 150L = 0.2%

2. Add subsequent pieces of 1g until suitable concentration has been reached:

0.2% --> we add 1g --> 0.1% --> we add 1g --> 0.05%

0.05% is lower than 0.09%, hence sufficient!

Answer:  2g (B) .

bumpbot · Answer

Automated notice from GMAT Club BumpBot:

A member just gave Kudos to this thread, showing it’s still useful. I’ve bumped it to the top so more people can benefit. Feel free to add your own questions or solutions.

This post was generated automatically.

Chlorination is used in industry for the purification of water; 1gm so

Prep Toolkit

Top 5 GMAT Debrief Videos

615 to 715 in 15 Days (Ria's Strategies)

More than Quant/Verbal Abilities: Speed vs. Accuracy

745 with Only Self-Prep and GMAT Club

From 555 to 765: 210-point Improvement

Perfect 805 Score Debrief by Julia

My Rewards

GMAT Problem Solving (PS) Questions

Chlorination is used in industry for the purification of water; 1gm so

Prep Toolkit

615 to 715 in 15 Days (Ria's Strategies)

More than Quant/Verbal Abilities: Speed vs. Accuracy

745 with Only Self-Prep and GMAT Club

From 555 to 765: 210-point Improvement

Perfect 805 Score Debrief by Julia

My Rewards