How much does the mixture now weigh?

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.

4 Pounds...

Thanks
Manoj:)

how can u say 4 pounds?

who told u dat the percentage given is v/v or w/v or w/w ratio?
u hve assumed it as w/w ratio!!

90% of water = 18 pounds
10% of solid = 2 pounds

After a week,

This 10% of solid changes to 50% of solid = 2 pounds
so another 50% of water is also = 2 pounds

So total weight of mixture is 4 pounds

Thanks
Manoj:)

I know how u get 4 pounds!! bt my question is y u took it as weight ratio?
y it cant be a volume ratio ?
y u assusmed like dat!!

Hi Smart Coder,

The question is "How much does the mixture now weigh?" not "What is the volume of the mixture?"

And there is none of the value in the question speaks about volume??

I dont know why u got this question??? Anyway good thinking???

Thanks
Manoj:)

Look Manoj...I dnt know whether u hve studied chemistry ever..bt i was a good student of chemistry!! Question says mixture has 90% of water and it weighs 20 pounds...so cnt be possible dat mixture has 90 % of water as volume and mixture weighs 20 pounds...and den later mixture has 50% of water as volume and now he is asking weight of mixture!!

There is nowhere written in question that 100 gm of mixture has 90 gm of water ( 90 % ) like u assumed!! There can b another deduction like mixture is 100 ml and 90 ml is water ( still 90 %) !! wt do u think now?

Hi Smart coder,

"Question says mixture has 90% of water and it weighs 20 pounds...so cnt be [B]possible[/B] dat mixture has 90 % of water [B]as volume[/B] and mixture weighs 20 pounds..." - From Your post

In the question can u tell me the way how u assumed as Volume... Imagine the same way i assumed as weight. But still Volume is a possibility but it will not give u the answer. If u assume it as weight then u will get the answer. I have studied chemistry very well and i know where to use that too.

And the question is "How much the mixture weigh now??" If u want then prove me by assuming the 90% of water as volume and find the weight.

Thanks
Manoj:)

Yes..we can find it..We know the density of water.
Suppose the density of water is d (pounds/ml)
Suppose mixture is 100 ml...90 ml water and 10 ml of other substance.
90 ml water weighs 90d.
weight of 10 ml of other substance = 20 -90d.
weight of 10 ml of water = 10d.

in new mixture there is 10 ml of water and 10 ml of other substance.
i.e 20- 90d + 10d = 20 - 80d

Density of water is approx 1 gm/cm^3
Convert it into pound / ml and u will get the solution.
Got it???

Hello,

Suppose the density of water is d [B](pounds/ml)[/B] Density of water is not 1 pound per 1 ml.
90 ml of water weighs 90d is wrong. (how u assumes 90ml = 90(pounds/ml))

weight of 10 ml of other substance = 20 -90d is also wrong (20 in pounds and 90 in pounds/ml) because the density of other substance is different from the water that we dont know...

Thanks
Manoj:)

If 1 ml weighs d pounds ...so 90 ml will weigh 90d pounds( improve ur basics)
Density of water is 1 gm/cm^3, so we can convet it into pounds / ml
unit of 90d is pounds not pounds/ml. I think ur basics are not clear.
and then wight of 10 ml of other substance has to be 20 -90d.

i think u will require much more clarification of units.

if density is d pound/ ml
and volume is x ml

Then weight of x ml is x(ml ) * d (pound/ ml ) = x*d pounds

Hello,

Because u have written d as the density of water thats y asked the question...

Now the question is what is the value of d???
Whether anywhere in the question gives the value of d...

Thanks
Manoj:)

no, but we know it is 1 gm/ cm^3... so we can find it in pounds / ml which is equal to 0.002205 pounds/ ml precisely.

"if density is d pound/ ml
and volume is x ml

Then weight of x ml is x(ml ) * d (pound/ ml ) = x*d pounds"

How u got the unit of volume as "ml" But the unit of volume should be in cubic
so x(ml^3) * d(pound/ml) = x *d (ml^2 * pound)

Thanks
Manoj:)

oh god..i cnt believe dis now...u dnt know dat ml( mililitre) is a unit of volume and 1 cm^3 = 1 ml.

Ok thankz man... Now i remember wat i studied 6 years back... still i'm feeling that there is a relationship between your answer and my answer...

thanks
Manoj:)

There cant be a relation MAnoj...u hve assumed the ratio by weight and i hve assumed ratio by volume. In these type of questions there should be explicit mention of the type of ratio.
In volume ratio..density has a role to play.

Hello! Does anyone hear me? I was amazed by how many riddle sites would have the wrong answer of 12 lbs. So your close to showing this answer is wrong. I would like some help in showing where they are going wrong.

if answer is 12lbs then we have
20-80d=12 and so d=.1 lbs/ml of water.
as you pointed out this it not the density of water (which is equal to 0.002205 pounds/ ml precisely.) so twelve is wrong but how did they get twelve?