Question

100 plates served. The 1st waiter turnes every plate upside down. Then another waiter turned every other plate. Then the 3d waiter turned every 3d plate. The 4th turned every 4th plate, and so on, until the 100th waiter turned the 100th plate. At the end how many plates are placed normally?

Answer 1

At the end of the 100th turn, you have 90 plates placed normally.

Explanation:

Not sure if there is a formula to calculate this. I plugged the data into a spreadsheet and created formulas to turn every plate, every 2nd plate, every 3rd, .... to every 100th plate.