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Math & Logic · Ages 18+

Riddle #204

What comes next in this sequence, and why: 1, 11, 21, 1211, 111221, ?

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Answer: 312211

Each term describes the previous one aloud ("look-and-say"): 111221 is "three 1s, two 2s, one 1," which becomes 312211.

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More Math & Logic riddles for Ages 18+

#205 You have 12 identical-looking coins, and exactly one is counterfeit — either heavier or lighter than the rest, but you don't know which. Using a balance scale, what's the minimum number of weighings needed to both identify the counterfeit coin and determine whether it's heavier or lighter?
#206 Four people need to cross a rickety bridge at night, sharing one flashlight that must be carried on every crossing. The bridge holds only two people at a time, and a pair moves at the slower person's pace. Crossing times are 1, 2, 5, and 10 minutes. What is the minimum total time for everyone to cross?
#207 If it takes 5 people 5 hours to paint 5 identical fences, how long does it take 10 people to paint 10 identical fences, at the same steady rate?

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