Math II Probability

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1

<p>Hey guys, i was running through my PR Math II book, and hit a question that i could answer, but didn't quite understand.</p>

<p>"If a fair coin is flipped three times, what is the probability that the result will be tails exactly twice?" the answer was 3/8. </p>

<p>I know how to get the answer by writing out hht, tth, etc. but wanted to know how to solve it formulaically. </p>

<p>e.g. i know how to find probability of 3 heads, being 1/2^3, but with this only asking for exactly 2 tails out of 3, i don't know how to set it up!</p>

<p>thanks a lot in advance guys!</p>

2

<p>All the possibilities are 2<em>2</em>2=8.
The possiblities occcurs thrice:
HTT
THT
TTH
Therefore the answer is 3/8.
You check the Tricky mathii questions (NOT QUESTIONZZ) thread . I posted a similar question and some guys broke it down for me. So if my explanation is not good enough, go to this thread</p>

3

<p>I just write them out because on this test especially, it is better to be accurate than fast.</p>

4

<p>n C r [(probability of success)^r] [(probability of failure)^(n-r)], i believe, yay for AP stats ;p</p>

5

<p>on the TI 83/84:</p>

<p>2nd vars (dist)</p>

<p>binomial pdf (3, .5, 2)</p>

<p>=.375</p>

<p>=3/8</p>

<p>3 is the total number of flips, .5 is the chance of getting tails, 2 is the number of tails you want.</p>

6

<p>Thanks guys. Stanford, i checked that thread and i suppose it really is best to write the possibilities out when dealing with the various coin questions.</p>

<p>:(</p>

<p>Thanks a lot though guys!</p>