This is one of the best puzzles I have posted, and would highly encourage all of you to try it – kids as well as adults.
Four honest and hard-working computer engineers are sipping coffee at Starbucks. They wish to compute their average salary. However, nobody is willing to reveal an iota of information about his/her own salary to anybody else. How do they do it?
Alok Goyal's Puzzles Page 
Think the first guy can take a random large number x and add his salary to it , he passes that new number to the next guy who does the same. When the first guy finally gets a final number he subtracts the large number he added initially and divides by four
Take any 4 nos whose sum is 0. Write them on chits. Randomly give one nos to each. He/she needs to add that nos to his/her salary and give the result to the group. Average of these nos will be average of their salary.
Many Solutions are possible …
– Average is nothing but Addition of age/ 4 , Only first guy can have one big number say in thousands and remember it, Add age and whispers the final number to 2nd one, he can add his age to a number and can whisper the second number to 3rd person and 3rd once can do same and tell it to 4th one, 4th one can do the same and whisper to 1st one who knows the big number initially added, 1st one can subtract the added number from the final number and divide it by 4 to get the average ….
– addition can also be replaced with subtraction as well…..
– other solution could be to divide everyone’s age in three different integers for example a+b+c = age of 1st, these three numbers can individually be whispered to rest three engineers …. all the other engineers can also do the same .. they all can add the numbers told by others .. later all of them can sum up all the numbers they have and divide it by 4