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# A substance is 99% water. Some water evaporates, leaving a substance..

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Manager
Joined: 30 May 2012
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Location: United States (TX)
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A substance is 99% water. Some water evaporates, leaving a substance..  [#permalink]

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20 May 2017, 18:26
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A substance is 99% water. Some water evaporates, leaving a substance that is 98% water. How much of the water evaporated?

Source is MathForum.org.
Per Posting Rule 12. ONLY Questions from Reliable GMAT Sources are Allowed on the Forum! Please do NOT post questions from random math books and web sites. Apologies if the post does not comply with the rule.
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Re: A substance is 99% water. Some water evaporates, leaving a substance..  [#permalink]

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20 May 2017, 18:44
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Blackbox wrote:
A substance is 99% water. Some water evaporates, leaving a substance that is 98% water. How much of the water evaporated?

Source is MathForum.org.
Per Posting Rule 12. ONLY Questions from Reliable GMAT Sources are Allowed on the Forum! Please do NOT post questions from random math books and web sites. Apologies if the post does not comply with the rule.

initial 1% volume of substance (ie apart from water ) is same after evaporation ,which now constitutes for 2% of final substance
thus as per rule given by karishma
C(initial) * V(initial) = C(final) * V(final)
where C=concentration
V= volume
Let X amount of water evaporates from 100 gms
then
1% of 100 = 2% of (100-x)
1= 2 - x/50
thus x = 50
now 50 is what % of 99
ie 50 = A*99/100
A= 50.55%

hope it helps!!!!!!!
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Re: A substance is 99% water. Some water evaporates, leaving a substance..  [#permalink]

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20 May 2017, 21:27
Thanks, Rohit for answering. I was also wondering, given that I have been following the table method (https://www.youtube.com/watch?v=JVlfQEhzLMM), if you could be so kind to rephrase the solution accordingly?
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Re: A substance is 99% water. Some water evaporates, leaving a substance..  [#permalink]

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20 May 2017, 21:58
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Assume substance is 100 ml (milli-litres). So out of 100 ml, 99 ml is water and 1 ml is substance.

Lets say x ml water evaporates.
So remaining water = (99-x) ml
remaining substance = 1 ml

We are given that now water =98%, so substance =2%
Thus ratio water/substance = 98/2
Or (99-x)/1 = 98/2
Solve to get x=50

So water that evaporated in ml = 50 ml
Water that evaporated in % = 50/99 * 100 = 50.5%
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Re: A substance is 99% water. Some water evaporates, leaving a substance..  [#permalink]

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20 May 2017, 22:10
amanvermagmat wrote:
Assume substance is 100 ml (milli-litres). So out of 100 ml, 99 ml is water and 1 ml is substance.

Lets say x ml water evaporates.
So remaining water = (99-x) ml
remaining substance = 1 ml

We are given that now water =98%, so substance =2%
Thus ratio water/substance = 98/2
Or (99-x)/1 = 98/2
Solve to get x=50

So water that evaporated in ml = 50 ml
Water that evaporated in % = 50/99 * 100 = 50.5%

I am pretty sure you lost me with the solution. Honestly, I hate to rewire my approach to mixture problems especially when I completely grasped the "Table" way of solving this type of questions. That is the reason why I asked if someone could solve it using the approach from the link (Khan Academy).
Re: A substance is 99% water. Some water evaporates, leaving a substance.. &nbs [#permalink] 20 May 2017, 22:10
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