Potatoes

[JohnBS]

Entertainment for Christmas.

Fred brings home 100lbs of potatoes, which are 99% by weight water.
He leaves them in the boiler-room overnight, and in the morning they contain 98% water.
How much do his spuds weigh now?

(Imagine this is a question on University Challenge, no pencil & paper!)

[Murphy Maniac]

Is it 99lbs? My logic being (probably incorrect as my brain has just turned to mush thinking about it): If it was 99% water by weight to start with that means that 1lb was potato. Assuming the potato has not evaporated there must still be 1lb present. We are still talking about a percentage of his original 100lbs of potatoes which now contains 98% water ie 98lbs of water plus the original 1lb of potato making 99lbs. Do I pass my starter for 10?
Geoff.

[JohnBS]

Geoff: your first couple of sentences are flawless.....
John

PS if someone works this out, please don't post the answer straight away. It's fun because it's counter-intuitive, and your "system 1" brain* has a stab at it and normally gets it wrong
*Thinking, Fast and Slow" Daniel Kahneman (Noble Prize winner)

[Murphy Maniac]

Your puzzle is giving me a sleepless night and has made it necessary for me to get up and share my latest thoughts!
Following on from my previously flawed logic; If 98% of his original 100lbs of potatoes is now water, that implies that the other 2% must be potato, suggesting 2lbs of potato which is clearly impossible as this suggests that the original 1lb of potato has now become 2lbs.
I'm going back to bed Ahh.....

[Murphyv310]

Hi.
That's two puzzles on this Forum that have done my nut in. So is the problem lack of sunlight, dodging rubbish Christmas drivers or lack of brain power. Think I'll go back to bed and dream about Turkey sandwiches.

[peter scott]

We are still talking about a percentage of his original 100lbs of potatoes
Geoff.

No, we are now talking about 98% of something less than 100lb.
Don't ask me what that something less is though.

Peter

[Doodlebug]

The water did weigh 99 lbs (99% of 100lbs)

The water reduced its weight by 1 % of 99lbs which is .99 lbs.
Water weight is now 98.01 lbs.

So potatoes now weigh 98.01 plus original potato material of 1 lb = 99.01 lbs.
Any good?

[ppppenguin]

If 98% is water then 2% is spud. Hence weight of water is 98/2=49 times weight of spud. Since there's 1lb of spud that makes 50lb altogether.

[JohnBS]

So, Jeffrey, you're suggesting that his 100lbs of spuds, @ 99% water content turned into 50lbs @98% content overnight?

John

[Murphy Maniac]

Doodlebug, you pre-empted me, my latest thought following a sleepless night is 99.01lbs. This problem has now gone viral amongst my friends this morning causing grief across Liverpool. The latest suggestion is 97.02lbs, but I have no idea how he arrived at this figure.