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Riddle me this: |
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Feb19-09, 11:49 PM
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#1
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Shackleford is
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Riddle me this:
Mary was working in a chemistry lab with a mixture of chemicals that was 90% water and weighed 20 pounds. After returning to the lab from a weekend break, she calculated the mixture was now 50% water. How much does the mixture now weigh? For purposes of this puzzle, assume the non-water part of the mixture was not affected by evaporation.
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Feb20-09, 12:05 AM
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Last edited by MathematicalPhysicist; Feb20-09 at 12:20 AM..
#2
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MathematicalPhysicist is
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Re: Riddle me this:
Isn't there a brain teaser subforum here?
Anyway, x-other substance. y-water, x,y in pounds.
0.9(x+y)=20
10y/9=20
y=18
18*0.5=9=y
First the mixture had 200/9 pounds after that it lost 11 pounds, so it weighs 200/9-11 or so I think, I myself just woke up an hour ago. (-:
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Feb20-09, 12:21 AM
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#3
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MathematicalPhysicist is
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Re: Riddle me this:
Can you show your equations?
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Feb20-09, 12:40 AM
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#4
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Shackleford is
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Re: Riddle me this:
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Feb20-09, 12:55 AM
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Last edited by Shackleford; Feb20-09 at 01:03 AM..
#5
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Shackleford is
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Re: Riddle me this:
In my sleep deprived state, I now have four pounds. lol.
10/9 x = 20 pounds
x = 18 pounds
10/1 y = 20 pounds
y = 2 pounds
It says the non-water part was unaffected so that would mean the 2 pounds of non-water is still there, correct?
Now, the mixture is 50% water or is a 1:1 ratio. Two pounds to two pounds? The mixture now weighs four pounds?
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Feb20-09, 01:07 AM
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#6
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MathematicalPhysicist is
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Re: Riddle me this:
At least I know your'e honest.
Ok, I'll try to find a way.
Mary was working in a chemistry lab with a mixture of chemicals that was 90% water and weighed 20 pounds. After returning to the lab from a weekend break, she calculated the mixture was now 50% water. How much does the mixture now weigh? For purposes of this puzzle, assume the non-water part of the mixture was not affected by evaporation.
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At first the water 20/0.9 pounds per percent
40 percent of the water evaporated meaning is gone, i.e 0.4*0.9 times 20/0.9 means
8 pounds were evaporated which means the water now weighs 12 pounds.
As I said the whole compound weighs 20/0.9 so if it loses 8 pounds it still should weigh more than 12 pounds, or am I way off here.
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Feb20-09, 01:10 AM
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#7
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Shackleford is
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Re: Riddle me this:
Originally Posted by loop quantum gravity
At least I know your'e honest.
Ok, I'll try to find a way.
At first the water 20/0.9 pounds per percent
40 percent of the water evaporated meaning is gone, i.e 0.4*0.9 times 20/0.9 means
8 pounds were evaporated which means the water now weighs 12 pounds.
As I said the whole compound weighs 20/0.9 so if it loses 8 pounds it still should weigh more than 12 pounds, or am I way off here.
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That's what I thought, too, but it just didn't seem logical to me. See if my previous post makes sense.
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Feb20-09, 12:56 PM
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Last edited by Topher925; Feb20-09 at 01:07 PM..
Reason: Whoops!
#8
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Topher925 is
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Re: Riddle me this:
Assuming your going by weight and not by stoichiometry, I got 4 lbs.
That website is wrong.
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Feb20-09, 01:09 PM
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#9
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lubuntu is
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Re: Riddle me this:
I remember these sorts of questions showing up in like 11th Grade Algebra or so, haha.
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Feb20-09, 01:28 PM
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Last edited by Topher925; Feb20-09 at 01:38 PM..
Reason: Crappy graph.
#10
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Topher925 is
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Re: Riddle me this:
Yea, its definitely 4lbs.
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Feb20-09, 01:38 PM
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#11
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Gokul43201 is
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Re: Riddle me this:
Here's your mistake:
Originally Posted by loop quantum gravity
40 percent of the water evaporated meaning is gone
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The answer is indeed 4 lbs.
Originally Posted by Topher925
Assuming your going by weight and not by stoichiometry, I got 4 lbs.
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What is going by stoichiometry, and how would that give you a different answer?
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Feb20-09, 01:41 PM
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#12
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Topher925 is
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Re: Riddle me this:
If you go my moles of molecules instead of weight you will get a different answer do to the chemical and water having different densities.
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Feb20-09, 03:31 PM
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#13
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Shackleford is
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Re: Riddle me this:
Originally Posted by Topher925
If you go my moles of molecules instead of weight you will get a different answer do to the chemical and water having different densities.
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Yeah. I just made the assumption by weight. That stupid website probably has so many incorrect answers to the riddles.
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Feb20-09, 03:38 PM
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#14
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Gokul43201 is
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Re: Riddle me this:
Originally Posted by Topher925
If you go my moles of molecules instead of weight you will get a different answer do to the chemical and water having different densities.
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If you assume the percentages are in fact mole fractions (or volume fractions), then there is no way to solve the problem with the given data. You will additionally need the molecular weight (or density). So, in order to solve it, you have to assume the numbers are mass percentages.
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Feb20-09, 04:23 PM
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#15
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Topher925 is
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Re: Riddle me this:
Originally Posted by Gokul43201
If you assume the percentages are in fact mole fractions (or volume fractions), then there is no way to solve the problem with the given data. You will additionally need the molecular weight (or density). So, in order to solve it, you have to assume the numbers are mass percentages.
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Or you could just claim that the riddle is a trick question and not bother answering it.
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