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September 24th, 2009, 10:05 PM   #1
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Simple probability problem...

...for some, but not for me.
Quote:
If five coins are flipped simultaneously, what is the probability that at least two will be heads?
Could someone please post the formula for solving this particular problem?

Thanks and cheers,
Tyster
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September 24th, 2009, 10:24 PM   #2
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Re: Simple probability problem...

You know the probability of each one landing heads is 1/2. (And tails is 1/2)
And when we want "x to happen" AND "y to happen" AND ..., we have to multiply the probabilities.
And finally, when we want "x to happen" OR "y to happen" OR..., we have to add the probabilites.

For exactly 2 heads, we need 3 tails. So, what is the probability of 2 heads, and 3 tails?

But we need *at least( 2 heads:

So, we can add: P(exactly 2 heads) + P(exactly 3 heads) + P(exactly 4 heads) + P(exactly 5 heads).

Or, we can realize that P(X happens) = 1-P(X doesn't happen),
and we can add: P(0 heads) + P(1 head), and subtract that from 1.

There's a slightly more advanced way of doing this (using combinations), but this is a simple enough problem that it's more trouble than it's worth.
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September 27th, 2009, 04:48 PM   #3
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Re: Simple probability problem...

Thanks for the explanation. Cheers.
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