A number theory problem by Joshua Cortright
Elliot had 100 light bulb switches. He invited 100 friends over and assigned them a different number from 1 to 100. All of the switches were off. The person who was number 1 flipped every 1st switch, the person who was number 2 flipped every 2nd switch, and so on until the person who was number 100 flipped every 100th switch. How many switches were on at the end?