People stand in a circle. Start from any person & they tell the person on the right the previous sum (0 at the start) added to the unit digit of their Age OR the ten’s digit of their Age multiplied by 10 (E.g, If Alice is of age 45 & starts, she tells Bob either 40 or 5). Continue this circle twice & by the end, the starting person knows the sum of all the ages. Dividing this by the number of people will give the average age.

]]>1. Everyone chooses a set of N numbers that add up to their age, randomly over some large range.

2. They privately communicate each number to a different person, including “communicating” one number to themselves.

3. Everyone announces the sum of the numbers they got.

4. The sum of the announced numbers is the sum of the ages.

With no trusted third parties, you can calculate the ages such that (N-1) people must collude to reveal an actual age (which is logically the best you can do!).

This is a motivational puzzle in: https://en.m.wikipedia.org/wiki/Secure_multi-party_computation

The sillier answers:

They could also use the ‘recently’ discovered homomorphic encryption.

Of course anyone could lie, so they secretly know the average as long as no-one else does—and no amount of collusion will discover their true age!

]]>Repeat procedure with remaining numbers left with each person and other 2 handlers until every person was handler.

It seems to me that to know age of other or others all have to collide. But then they could just share their ages. ]]>