(Answer and then post your own!)

You have 9 identical-looking, identical-looking, and generally indistinguishable eggs and a fair balance scale. Eight of the eggs weigh the same, and one is slightly heavier (but you can't tell just by holding it). You are allowed to use the scale a maximum of two times. How do you determine which egg is the heavy one?


(If you've already heard of a brain teaser, don't spoil it right away! This particular one is a pretty popular one for interviews (especially finance and consulting interviews) so you might have heard it before.)

Heard this one.

Split eggs into 3 groups of 3. Weigh two of the groups, if one side goes down, heavy egg is in that group, if the scale is even the heavy egg is in the group you didn't weigh. Set aside the two groups that you know don't have the heavy egg, and weigh two of remaining eggs on the scale. Whichever one goes down is the heavy egg. If neither does, the third egg you didn't weigh is the heavy one.

Or you could do something clever like roll them all along the floor... the heavy one will have more friction and roll less, assuming you can roll all the eggs with exactly the same force.

I'll post one since MZZT didn't.

This one is the same as Pommy's except:
You have 12 eggs.
11 eggs weigh the same.
1 weighs differenty (either heavier or lighter)
You can use the scale a maximum of 3 times.

Which egg weighs differently?

MS asked that one at an interview for someone I know.

I thought Microsoft stopped using puzzles a long time ago. When they found out that people who can come up with overcomplicated solutions to non-problems don't make good software engineers.

Here's a brain teaser:

Suppose you are an assistant to the CEO of a large company. The board of directors has 16 members. You need to organize meetings in groups of 4:

- every director meets with every other director in a meeting.
- no two directors meet twice.
- at every given time, everybody attends a meeting.

use vectors in a Galois field

They still ask the "how would you design X for Y group of people" questions to PMs.