# Riddle me this:

1. Feb 19, 2009

### Shackleford

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.

2. Feb 19, 2009

### MathematicalPhysicist

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. (-:

Last edited: Feb 19, 2009
3. Feb 19, 2009

### MathematicalPhysicist

4. Feb 19, 2009

### Shackleford

5. Feb 19, 2009

### Shackleford

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?

Last edited: Feb 20, 2009
6. Feb 20, 2009

### MathematicalPhysicist

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.

7. Feb 20, 2009

### Shackleford

That's what I thought, too, but it just didn't seem logical to me. See if my previous post makes sense.

8. Feb 20, 2009

### Topher925

Assuming your going by weight and not by stoichiometry, I got 4 lbs.

That website is wrong.

Last edited: Feb 20, 2009
9. Feb 20, 2009

### lubuntu

I remember these sorts of questions showing up in like 11th Grade Algebra or so, haha.

10. Feb 20, 2009

### Topher925

Last edited: Feb 20, 2009
11. Feb 20, 2009

### Gokul43201

Staff Emeritus
The answer is indeed 4 lbs.

What is going by stoichiometry, and how would that give you a different answer?

12. Feb 20, 2009

### 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.

13. Feb 20, 2009

### Shackleford

Yeah. I just made the assumption by weight. That stupid website probably has so many incorrect answers to the riddles.

14. Feb 20, 2009

### Gokul43201

Staff Emeritus
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.

15. Feb 20, 2009

### Topher925

Or you could just claim that the riddle is a trick question and not bother answering it.