Question:
You have 100 coins, each either heads or tails. You can't see the coins, but you can flip them over. What's the minimum number of flips needed to guarantee that all coins are heads?
Answer:
20 flips
Explanation:
Flip half of the coins (50) over. Then, for each of the remaining 50 coins, flip it over if it's tails. This guarantees that all coins are heads because any heads coins will not be flipped over in the second step, and any tails coins will be flipped over first and then flipped over again in the second step.
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