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A Math Problem From Singapore Goes Viral: When Is Cheryl’s Birthday?

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A Math Problem From Singapore Goes Viral: When Is Cheryl’s Birthday?

Postby Tempo Gain » 15 Apr 2015, 02:18


"Being alive is the meaning." -- Joseph Campbell

"Well I don't guess it has to be, but that's the way we like it." -- Roger Waters

There's only one thing left for me to do momma I gotta ding a ding dang my dang a long ling long
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Re: A Math Problem From Singapore Goes Viral: When Is Cheryl’s Birthday?

Postby Blaquesmith » 15 Apr 2015, 08:00

I remember that my math teacher at school was specially fond of logic problems, and each week he would give us a couple to solve.

For example:

You have 10 crates of gold coins that were salvaged from an old shipwreck. All crates have the same appearance and the coins look exactly the same. However, one of the crates has been swapped by a cunning thief and all the coins in that crate are almost perfect replicas.

The only difference between the replicas and the true thing is 0.01 grams: the good ones weight is 10g, and the bad ones 9.99g.

In order to find out the thief, you have a one-plate digital scale that works as follows: you load the plate, you press a button, and it gives you the weight of what was in the plate in the instant you pressed the button. You also have a felt-tip marker.

The solution here is finding out which crate is the fake one, which seems pretty obvious.

However, the big price is for the person that can find out which one is the fake using the scale as few times as possible.

Spoiler: show
The best answer is that you only need to use the scale once.

You use the marker to assign a number to each crate, you pick as many coins from each crate as its number, and write the number on them.
Then you gather all the coins together and weigh them. The number of centigrams missing will give you the number of the fake box.
"Roda el món i torna al Born!" - famous saying from Barcelona.
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Re: A Math Problem From Singapore Goes Viral: When Is Cheryl’s Birthday?

Postby Abacus » 15 Apr 2015, 10:07

I had to think about Cheryl's birthday for 15-20 minutes the other night while I was blogging. There's a moment when the lightbulb goes off and it gets easy. I think these are good for kids to work on but I don't like how much they are incorporated into HW and tests.
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