Fruit Machine Chat

A really simple question when you work it out, but might puzzle a few for a while.

You have 9 ball bearings, 8 weigh exactly the same, and the 9th's weight is slighty more.

You have a old fashioned scale (i.e. with 2 plates, where if you put a ball bearing either side it will balance).

You can only use the scale twice, how do you use the scale so that you can determine without fail which is the ball bearing that weighs a slightly different amount?

Put three balls on each side of the scale, that tells you which group of three contains the heavy ball. If the scale is balanced, the heavy ball is in the group not on the scale, otherwise it's in the group on the lowest side of the scale.

Put one ball from that group on each side of the scale, that tells you which ball is the heavyist. If the scale is balanced, the heavy ball is the one not on the scale, otherwise it's the ball on the lowest side of the scale.

Correct, amazing though how many people don't think to split it by 3's though.

I was asked it on a job interview once, it made me stumble for a bit (although I did get the job).

Yeah, I've had interviews like that. I'd ask one of the questions here, but they're a bit specific.