You and your two friends are working in a multinational company. How can you three find out the average salary of you all without disclosing your own salary to the other two?
A famous swimmer can swim downstream in a lake in exactly 40 minutes with the lake current.
He can swim upstream in that lake in exactly 60 minutes against the lake current.
The length of the lake is 2 km.
How long he can cover the distance of one side at a still lake with no current?