11.12.2008

Brain Teaser, #8.

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.]

8 comments:

george said...

I'm going with 4 lbs.... I could show my work, but I have a feeling I'm wrong, so what would be the point?

.mk. said...

G, this is not like you, i'm disappointed. Next guess!?!

Anonymous said...

ok...well this caused a marital fight...but my husband said 4 lbs, then decided you couldn't determine the weight unless you knew the density of the other substance. I said 11 lbs.

abbyjo said...

right - you don't have enough info.

.mk. said...

Yikes! I dont' want to start fights, just finish them. It's by volume, not by weight.

george said...

If it were by weight, my first answer (4 lbs) would be right. If you say it's by volume, then you don't have enough information without knowing either the density of the other substance, or the total volume of the mixture (either before or after the evaporation).

Marty, the method you told me to use the other night that gave you an answer of 12 lbs just doesn't make sense. First, a reducing the substance from 90% water to 50% water is not the same as reducing the total volume of it by 40%. And second, reducing the total weight by 40% would mean that the non-water substance was evaporating also, which the question says is not the case. If the volume of the non-water substance stays constant, the mixture will be more concentrated after the evaporation of some of the water, so its "weight per volume" would change. Which means that even if the volume DID decrease by 40%, the change in weight would be something different than 40%.

So since the only way to get an answer is to assume that the question should read that the mixture was "90% water by weight", I'm going to claim victory and go home!

.mk. said...

everyone's a winner.

.mk. said...

but please feel free to leave the author, jim taylor, a note on the website I found it on: http://riddles.com/brain-teasers/brain-teasers/the-chemistry-lab--2008080566541/

transplanted.chicagoan

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