How well do you think you can estimate something? How good are you at guessing the answers to problems? How fast can you do it? How accurate are your guesses? In this note, we're going to put your abilities to the test. And we're going to do that with a simple math problem. Are you ready?

I want you read the following problem carefully and (this part is really important) *not* do the calculations in your mind or on a piece of paper. I want you to simply *guess*. Also note down how fast you're doing the guessing. We're going to take that into consideration as well. Got that? Here comes the problem:

Suppose you have \(100\) pounds of potatoes. And these potatoes consist of \(99\) % water. You decide to leave the potatoes outside and let them dehydrate until they consist of \(98\) % water. Now the potatoes should weigh a little less than they were before. How much do they weigh now?

Okay, I want you to guess ... *now*! Don't do the math. Just guess [you'll miss out on something fun if you don't]. And you don't have to worry about being correct. You just have to get something close to the correct answer. Have you done that? Write your guess down.

So, how well do you think you did? What was your answer? \(99\) pounds? \(98\) pounds? \(90\) pounds? Prior to writing this note, I did a little survey on this problem with about \(20\) people. And the average answer was \(98.7\) pounds. Was your answer in this ballpark?

Surprisingly, the correct answer is \(50\) pounds! Yes, the weight decreases by \(50\)% because of the \(1\)% decrease of water! People find this answer very non-intuitive.

Here's a short solution of the problem. Since initially you had \(99\)% water, the water weighed \(99\) pounds while the non-water part weighed \(1\) pound. After the potatoes dehydrate, you're left with \(98\)% water and \(2\)% non-water stuff. Remember that the weight of the non-water stuff never decreases. So, \(2\)% of the new weight is equal to \(1\) pound.

\(1\) pound is \(2\)% of how many pounds? \(\boxed{50}\) pounds and that is our answer.

Were you surprised to see this answer? You're not alone. Your brain probably thought, "Hey, the water decreased by \(1\)%. The weight of the potato can't decrease that much." So, you went with an answer that wasn't too far off from \(100\) pounds.

Feel free to post what your guess was in the comments section. And if you have any feedback, I'll be happy to hear it.

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## Comments

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TopNewestI actually knew it because its an exact duplicate from a jelly problem on Brilliant I did a few days ago, level 2 algebra I think. A beautiful, nontrivial problem nonetheless.

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Exactly! I solved the same one!

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Oh! I didn't know that there was already a problem on this on Brilliant. Should have posted this earlier. By the way, I didn't make this problem. 'The Potato Paradox' is a simple problem with a very non-intuitive answer. Check out the Wikipedia article on it.

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lol.. everything isn't a paradox brother

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yea, saw that also. but if i haven't, my answer would probably be 95. HAHA

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Mind = Blown.

I almost argued you that decreasing 99 to 49 pounds doesn't make sense to say that you're decreasing by 1%.

Let me explained you why this works.

1/2 = 50%

2/3 = 66.66....%

3/4 = 75%

4/5 = 80%

..................

when we keep doing this, you'll see that when you increase by 1 by 1... the percentage increases. But it keeps decreasing the rate of increase and stuffs. So we need to increase the fraction a lottttt to make the percentage change by 1, as we see below.

49/50 = 98%, 99/100 = 99%

Hope I answered your question. You tagged #JustForFun, but I'm freaking serious about this! XD

Oh, I forgot my guess, but I know I can't get this answer, lel. XD

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Nice explaining on your part! I already knew why this works [I shared this note, remember? :) ]. Thanks nevertheless. Did you enjoy it?

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Yup! My mind's actually stunned for 2 mins. Now I know I can't use common sense any more. XD

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Did anyone else look at the answer like a million times and still think,"That is witchcraft"? Anyway, i thought this note is very fascinating. Great job!

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Thanks!

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Same here!!!!

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wat my brain hurts

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Still hard to believe it

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That is what makes this problem so cool. Even though it's pretty easy to solve, the answer is really non-intuitive and most people wouldn't come close to it if they guessed.

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I guessed 98 percent, interesting

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the weight will reduce by 50%....

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This problem can be solved in less than a minute.......

by using the concept of mixtures and alligations......

now, understand that the potato is made up of water and the mass.... the weight of mass in going to remain the unchanged.....

so as total weight is 100 pounds..... and 99% is water and 1% is mass..... the weight of water in the potatoes is 99 pounds and mass is of 1 pound.......

So to get the mixture of water and mass to get the potato....... with 98% of water.......

mix the two concentrations.... with weight of the mass is 1 pound after dehydration

concentration of WATER MASS

by alligation rule... solve .....

So, 50 pounds is the weight of the potatoes after dehyadration.............................hahahahahah

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I guessed 99.2

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It's really hard to belive..can someone carry out this experiment and check maths. ....

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nice trick

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me too solve it...

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I was wrong my answer came 97.02 lbs

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This problem made me awake since I have been sleeping while reading.......Yep I could hav done this

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owesome!

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99 pound

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first i got 98.7 gm but latter i find out it 50 gm.

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so 1 pound=2%means 50 pounds= 50*2=100%

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wow.. yah had 98,. perfect sense by ur solution,.

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a nice trick the brain plays with us

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ya its 50 pounds only........

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100x2%=2 100-2=98.

98.00x1%=.98.00 98.00-.98.00=97.07 97.07 +2=99.07 Ans

Md.Wahidur Rahman Khan.

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Yeah The Answer was tricky and Correct .. How I calculated was if 1% of 100 is weight of non water (say y) then how 2% of how much will be y ? it is 50 of course .. But there is something in the Question which causes (I think) to guess an answer closer to 100 .. and it works .. if some body reads it they will think it should be closer to 100 . . > " Now the potatoes should weigh a little less than they were before" .. when our mind reads little less it is ought to think a number closer 100 ..

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i thought it would be like 52% around

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Not surprising at all...

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98.5%

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99

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how can anyone GUESS answer for this? :P indeed you have to calculate!

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99.5

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49

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99 pounds

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1% =1lb(which is obvious){ the "potato only" content which will not change }ypotato only" content not changed }Log in to reply

thid is what i really require... although I got the answer somehow I need some systematic way to approach this problem... tq:)

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99.5

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my guess was 50.

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nice one

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I thought it was 98.

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i guessed 98%

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50 indeed is the answer. it's easier to visualize if you plug this in in a table. the trick here is to use percent by weight not percent composition

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interesting one... post some more ques. like this... :)

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really smart one...

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intersting man ..i was also wrong

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haha... very interesting question.... but I got the 50 lbs answer just after reading the question... (1/0.01=100lbs and 1/0.02=50 lbs) ... if 1lb potatoes are 2% of the total mass then the total mass ought to be 50 lbs... :D

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it's still confusing me because this is the first time I join with brilliant.org. but, I enjoy with that.

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Totally true. I saw a question just like this a while ago and I think I immediately thought 98

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Think it this way: There s a giant potato of 100 lb. So initially 99 lb was water. We wanna find the amount of water lost so that finally water will contribute to 98% of the weight. Let X be the weight of the water lost. Reduction in weight of water =99-X Reduction in total weight =100-X Now this must comprise 98% by weight. Putting in equation, ((99-X)/(100-X))*100= 98% Solving for X, we get X= 50 lb Which gives the remaining weight= 100-X = 50 lb

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Why mountains being close to sun than ground remain colder?

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99.9 The tiniest bit possible! Oh well, turns out I was wrong. I had to read it twice to understand, thought.

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Potatoes have seldom been a man's problem in this part of the world, I mean India, Pakistan, Bangladesh, Sri Lanka, Nepal and also include Middle East countries.

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99%

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i guessed 90 lbs

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Beautiful.. Never able to guess that it would be around 50%. If I would have taken my pen and pencil, then I would have got it correct. Not gud in guessing

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i thing 11 potatos....not fully correct i guess....:-p

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98

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ans is 98 is it correct

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yoyoyo my famalam sweats suck it

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Nah, this solution is wrong I think. If the water decreases maybe because evaportaion,etc. The non-water stuff will not be increased, this is why I say the solution is false. So the ratio of the potato before and now is 100:99, so the answer must be 99 pounds (in my opinion)

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The problem with your reasoning is that you consider the new, 98% weight of the water to be based on the initial weight of 100 pounds. It's incorrect; the water is 98% of the new weight.

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no man. 50 is correct. remember that the non-water part is always fixed and only the water content changes. the water has changed to 98%, which means the non-water is now 2% (regardless of this, its weight is still the same after drying). now, if 1 lb is 2%, then by ratio and proportion, 50 lbs is the answer.

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