I need some mental stimulation

[iclim4]

Words Of Wisdom said:

Kasz216 said:

Words Of Wisdom said:

Kasz216 said:
I've got a good math riddle. Either it will take you like, 5 minutes...(like it did me.) Or it could take a day or even longer to figure it out. (like it did everyone else in my highschool math class.)

You have 10 stacks of coins that each have 10 coins in each stack.

Each coin weighs 1 Kilogram. Except for one stack of coins that instead weighs 2Kilograms each.

You have a scale, however you can only use it once.

How do you figure out which stack has the coins that weigh 2 Kilograms?

One truth prevails.

Take 1 coin from the first stack, 2 coins from the 2nd stack, 3 coins from the 3rd stack...etc and put them all on the scale.

If they all weighed 1 kilogram each, the total weight would be 55 kilograms. The number of kilograms the weight is off corresponds to the stack that is 2 kilograms.

56 = 1st column.
57 = 2nd column.
...etc.

Ah yeah it would be 55 coins.  I had it as 45 coins in my mind for some reason.

But yeah... bingo.

I was the only person to get it right in my freshman geometry class... which was embarrising cause the teacher then made me explain the thing.  Ah memories.

 

45 is what you get by letting the first column be 0 and doing 1 through 9 for the rest. 

If the first column is the 2 kilogram one, then the scale is not off at all. ^_^

My favorite puzzles are the "Lateral Thinking" style of puzzles.  They make you think outside the box and challenge the assumptions that you often make without even realizing it.

Can't you just weigh them by hand?
You should be able to tell which stack weighs 10 kilograms and which weighs 20 kilograms.
That way, you don't even have to use the scale. =P