HardMath and Logic
100 people flip switches of multiples of their entry. How many stay on?
Riddle
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
Hint: It's a word with 5 letters, starting with "1".
Answer
10 bulbs
Why this is the answer
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.
This riddle is filed under Expert Riddles for Adults and rated Hard.
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