Expected value problem
There are 7 prisoners, and 7 rooms each with a switch. Assume all switch are off at first.
Now there is a fair coin in each room. Each prisoner goes to each room and flips the coin. If the coin comes up heads, prisoner changes the state of the switch. Else, prisoner moves to the next room and repeats. What is the average number of switch that are on when all 7 prisoners are done?