This week's puzzle courtesy of: Syvum.

A block of wood in the form of a cuboid 4" × 9" × 11" has all its six faces painted pink.

If the wooden block is cut into 396 cubes of 1" × 1" × 1", how many of these would have pink paint on them?

Three coworkers would like to know their average salary.

How can they do it, without disclosing their own salaries?

Note: Assume that everyone is honest in what they reveal.  

Answer

How about: Person A writes a number that is her salary plus a random amount (AS + AR) and hands it to B, without showing C. B then adds his salary plus a random amount (BS + BR) and passes to C (at each step, they write on a new paper and don’t show the 3rd person). C adds CS + CR and passes to A. Now A subtracts her random number (AR), passes to B. B and C each subtract their random number and pass. After C is done, he shows the result and they divide by 3.

As has been noted already, there’s no way to liar-proof the scheme.

It’s also worth noting that once they know the average, any of the three knows the sum of the other 2 salaries.

Courtesy of  Techinterview