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.

 PhysOrg.com science news on PhysOrg.com >> Hong Kong launches first electric taxis>> Morocco to harness the wind in energy hunt>> Galaxy's Ring of Fire
 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. (-:

Riddle me this:

The stupid website says 12 pounds.

http://riddles.com/brain-teasers/bra...2008080566541/

 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?

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.
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.

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

 Assuming your going by weight and not by stoichiometry, I got 4 lbs. That website is wrong.
 I remember these sorts of questions showing up in like 11th Grade Algebra or so, haha.
 Yea, its definitely 4lbs.

Recognitions:
Gold Member
Staff Emeritus
 Quote by loop quantum gravity 40 percent of the water evaporated meaning is gone
The answer is indeed 4 lbs.

 Quote by Topher925 Assuming your going by weight and not by stoichiometry, I got 4 lbs.
What is going by stoichiometry, and how would that give you a different answer?

 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.

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

Recognitions:
Gold Member