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January 27th, 2011, 04:03 PM   #1
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Probability question

Question : How many times should a coin be tossed so that the probability of at least one head is >= 99%

Attempt, straight from the definition :

P(N = n) = (1-p)^(n-1) * p, where p is the probability of success

0.99 = (1-1/2)^n * 1/2

0.99 = (1/2)^n

n = 0.0145

What am I doing wrong?
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January 27th, 2011, 04:31 PM   #2
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Re: Probability question

Originally Posted by tnutty
What am I doing wrong?
You're calculating the probability of flipping n coins, all but the last being tails and the last being heads. It's an entirely different problem.

You should set the odds of getting all tails to 1% and solve.
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