Hard
Math and Logic
100 people flip switches of multiples of their entry. How many stay on?
Choose an answer
A.50 bulbs
B.1 bulb
C.All 100
D.10 bulbs
Answer: 10 bulbs
A bulb ends up 'on' if its switch is flipped an odd number of times. Only numbers with an odd number of divisors (perfect squares) satisfy this.
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