A Jelly Problem

Algebra Level 2

Imagine you have giant jelly. The jelly weighs 100 pounds, and is 99% water by weight.

You leave the jelly out, and let water evaporate from the jelly until it reaches 98% water by weight.

How much does the jelly now weigh in pounds?


The answer is 50.

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22 solutions

Satyen Nabar
Mar 1, 2014

The answer is that the jelly would now weigh 50 pounds!

When the jelly weighs 100 pounds, it is 99% water. That means that it is 1% non-water, and this non-water component of the jelly must weigh one pound.

After evaporation, the non-water component accounts for 2% of the weight of the jelly. So 2% of the jelly weighs one pound. Scaling this up, we see that, if 2% weighs one pound, then the entire jelly must weigh fifty pounds.

Dear Rohit, lets look at it differently. Say there are 100 students in a room and 99% are girls. That means there are 99 girls and 1 boy. Now in order for the girls to be 98%, 50 girls would have to leave, leaving 49 girls and 1 boy, each student accounting for 2% of people in the room. Similarly, 50 pounds of water must evaporate in this example to make it 98% The weight of the solid component of jelly has remained 1 pound.! Its the 50 pounds of water that has evaporated.

Satyen Nabar - 7 years, 3 months ago

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this really helped me understand! thanx bro.

Laxman Selvam - 7 years, 2 months ago

that boy must be lucky :D

Umar Farooq - 7 years, 2 months ago

But according to law of conservation of mass,shouldn't the weight of the jelly remain 1 pound only after evaporation of water?

Rohit Nair - 7 years, 3 months ago

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This is ALGEBRA.

Joshua Ong - 7 years, 3 months ago

The jelly's weight includes the water's weight.

Like when you measure your weight, you also include your own water weight.

Calvin Lin Staff - 7 years, 3 months ago

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i agree

math man - 7 years, 2 months ago

after evaporating the weight of the jelly is the same but its precentage in the mixature will be different with removing water

Mohamed Elzoghpy - 7 years, 3 months ago

all of the water from jelly isn't allowed to evaporate we just have to find the weight of jelly at the point where water% by weight is 98%.

Umar Siddiquee - 7 years, 2 months ago

The equation created will be ( (99 - x)/ (100 - x) ) x 100= 98 . Since water and weight will decrease by the same amount because water is evaporating. The equation is solved by 9900 - 100x = 9800 - 98x. Hence 100 = 2x and x = 50. Please note that x is the amount of water lost. So weight of jelly equals 50 pounds and weight of water equals 49 pounds.

Shiva Shankar - 7 years, 2 months ago

1 pound of raw jelly + 99 pounds of water = 100 pounds . After evaporation : water = x pounds ; raw jelly = 1 pound .... so x/(x+1) = 0.98 => x=49 .... so weight of jelly = 49+1 = 50 pounds

Aveek Dutta - 7 years, 2 months ago

i dont understand this , can any one explan this ?

zarree khan - 7 years, 1 month ago
Sunny Patel
Mar 2, 2014

let consider new weight is X

X= 1 (solid) + 98%(X)

X=1+0.98 X

X=50

Let the decrease in water weight be x x . Since percentage of water weight was 99%, it weighs 99 pounds. A 1 pound decrease in water weight would correspond to a 1 pound decrease in overall weight. From provided information: 99 x 100 x = 0.98 \frac { 99-x }{ 100-x }=0.98 . Solving the equation yields x = 50 x=50 . Therefore, total weight now is 100 50 = 50 100-50=50 pounds.

awesome question

Sunny Patel - 7 years, 3 months ago
Finn Hulse
Mar 9, 2014

There is one thing I'd like to clarify in this problem. It shouldn't just be the water evaporating, because that's not the same thing as the mass being decreased. Anyways, the way to solve is by setting up the following proportion, where x x is the amount being taken away:

99 x 100 x = 49 50 \frac{99-x}{100-x}=\frac{49}{50}

Cross multiplying and rearranging produces

4950 = 4900 + x 4950=4900+x

Solving, x = 50 x=50 . Thus the amount of jelly left is 50 \boxed{50} .

the same powder that is i percent of the initial weight of the jelly can only be 2 percent of its final weight if the final weight is 50 pounds.

meher engineer - 7 years, 3 months ago
Ahmed Romih
Mar 27, 2014

nice problem

Nilangini Gupta
Mar 26, 2014

The initial ratio of water : jelly is 99 : 1

now let x be the weight of the water that dries up.

The new ratio is (99-x)/1 = 98/2

solving this, x = 50

so new weight of water is 99-50 = 49

and the weight of jelly remains 1

so, total weight is 49+1 = 50

Shiva Shankar
Mar 22, 2014

The equation created will be ( (99 - x)/ (100 - x) ) x 100= 98 . Since water and weight will decrease by the same amount because water is evaporating. The equation is solved by 9900 - 100x = 9800 - 98x. Hence 100 = 2x and x = 50. Please note that x is the amount of water lost. So weight of jelly equals 50 pounds and weight of water equals 49 pounds.

Sabeel Mahmood
Mar 20, 2014

This one is similar to the discussion a week back, taking potatoes in place of jelly, it took me reading twice to understand it and it is cool. 99% goes to 98%, leaving 1% weight (1 pound initially) to 2% (that is still 1 pound). total is 100%, and 1 pound equals 2% so 100/2 = 50 lbs.

Ashtamoorthy Ts
Mar 19, 2014

the following tabulation works: nw is nonwater, w is water all weight being in pounds (nw, w, jelly) = (1, 99, 100) before and =(1, x, 1+x) after; since after, it is 98% water by weight, one has (1+x)/x = 100/98 whereby x=49 and jelly weighs 1+x = 50 pounds.

Shriya Agarwal
Mar 17, 2014

Initially, the jelly had 99% water or we may say 1% of other substances.. i.e., 1pound.. Later when some of the water evaporated, mass of the other substances would still have been 1 pound only.. now this time water constitutes 98% of the total mass , therefore, the other substances made 2% of new mass (say x) i.e., 2% of x = 1 pound 2/100 * x = 1 pound x = 50 pound

Kunal Mandil
Mar 15, 2014

as it is given 99% water i.e. 99 pounds of water and 1 % jelly substance i.e. 1 pound jelly substance.

now,

let x be the about evaporated, therefore, amount of water left = 99-x and total amount of jelly = 100-x then

99-x/100-x = 98 ............(given)

solving it we get x=50 total weight of jelly left = 100-50 = 50 pounds

Kai Dettman
Mar 13, 2014

The weight of the gelatin is 1 pound (1% of 100). If the water percentage goes down to 98%, it is now 49/50 of the jelly. The rest (1/50) is gelatin, and it weighs 1 pound, so 50/50 must weigh 50 pounds.

Govind P
Mar 12, 2014

Let x be the quantity of water after reduction. then: x/(1+x)=98% where 1 represents the dehydrated weight of jelly.(1-99% * 100pt.) Solving we get x = 49, so, Water contend + Dehydrated weight of jelly = 49 + 1 = 50pt.

Sudhir Jha
Mar 12, 2014

Initially in 100 pound jelly there was 99%water i.e. 1 pound solid. Now let weight of water be 'x' pound x/1+x = 98% 2x =98 x=49 1(wt. of solid)+x( wt. of water) =50pound

Dpk ­
Mar 12, 2014

The weight of Jelly = Weight of Water + Weight of Solid. let J = weight of jelly let W = weight of water let S = weight of solid since J = 100 and it is 99% water by weight, then W = 99, and S = 1. since weight of solid is constant, the formula for %mass of water is W/W+1 solve for W in the equation (W/W+1) = 0.98, you get W = 49. since J = W+S, then J = 49 + 1 = 50

Saurav Singh
Mar 12, 2014

jelly has 99 parts water and 1 parts mass. mass will remain constant i.e will not be altered after evaporation. so, x being new parts of water.

X/(X+1)=0.98 X=49. Hence,New weight = 50.

Kawsar Mamun
Mar 12, 2014

Among 100 pounds 99% is water.so water =99 pounds and non-water=1 pound.After evaporating non-water will remain 1 pound.Let, the jelly now weighs X pound,so 98%X+1=X.solving above equation X=50

Aditya Joshi
Mar 12, 2014

Since the Jelly is 99 % 99 \% water by weight, the remaining 1 % 1 \% , that is 1 100 × 100 = 1 \dfrac{1}{100} \times 100 = 1 pound \text{pound} is stuff that cannot be evaporated. That will remain constant.

After evaporation, let the content of Jelly by x x . Now, the Jelly is 98 % 98 \% water by weight.

Hence,

98 100 x + 1 = x \Rightarrow \dfrac{98}{100}x + 1 = x

Solving this, we get x = 50 \boxed{x = 50} .

Pat Nelson
Mar 10, 2014

2% of the weight is made up of solids, we know from the first section that the solids weigh 1 pound, so if 2% of the weight is 1 pound, 100% of the weight is 50 pounds

Subham Gupta
Mar 10, 2014

The amount of Solid is going to remain same before and after because it is not evaporating . so , it is 1% by weight before i.e. 1 Pound and at last it becomes 2% by weight i.e. now 2% is equal to 1 pound . Hence , weight now becomes 50 Pounds .

let x = pound lost

since 99% of 100 = 99, the initial amount of water must be 99 pounds, and the remaining 1 lbs must be the other content. since we are looking for the amount of water evaporated, 99-x = .98(100-x) ; x = 50 ; 100 - 50 = 50

The key to the answer is that the weight of the non-water part of the jelly will remain constant.

Initially, 99% is water. So within 100 pounds, 99 pounds are water, 1 pound is non-water.

Next, after evaporation, let the weight of the jelly be x.

So, 0.98x = water and 0.02x = non-water

As weight of water and non-water part is constant,

0.02x = 1 x = 50

So, weight of jelly after evaporation of water = 50 pounds.

1/x + 98%/100% = 1 1/x + 49/50 = 1 1/x = 50/50 - 49/50 1/x = 1/50 x = 50

Bonz Libiran - 7 years, 2 months ago

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