Ash Wyllie wrote:
A private pilot has a four-seat plane, and he's offered to take three
friends up for a flight. To do his load and fuel calculations the pilot
needs to know the combined weight of his three passengers. Now, the
three passengers are sensitive about their weight, and none of them will
let anyone else know how much he weighs. And no scale at the flying club
is big enough to weigh more than one person at a time. How does the
pilot quickly get the accurate combined weight of the three passengers?


This one has beeen aroound before. What you do is whisper a random number to
person 1. He adds his weight to that number and whispers the result to person
2. He and person 3 dothe same. Person 3 tells the pilot the result of his
calculation. The pilot then subtracts the original random number and comes out
with the total of the passenger's weights.

I like that!
Simple and elegant.