Khan/khan-exercises — Issue #14685: Probability 1 - Probability 1
Author: KhanBugz
Created: 2012-02-26 13:23:02+00:00
State: open
Labels: ie8, other, win7

Reporter: floatinggardenia@gmail.com

I don't understand the following problem. Where did the "11" come from?

A fair coin is flipped four times. What is the probability of getting heads at least twice? Write your answer as a simplified fraction.
There are 2⋅2⋅2⋅2=2 4 =16 2 \cdot 2 \cdot 2 \cdot 2 = 2^{4} = 16 possibilities for the sequence of flips.

The possibilities are HHHH, HHHT, HHTH, HHTT, HTHH, HTHT, HTTH, HTTT, THHH, THHT, THTH, THTT, TTHH, TTHT, TTTH, and TTTT.

There are 11 favorable outcomes: .

The probability is 11 16 \displaystyle \frac{11}{16}.

[probability_1.html?seed=184&problem=2](http://sandcastle.khanacademy.org/media/castles/Khan:master/exercises/probability_1.html?seed=184&problem=2&debug)

[Answer timeline](http://sandcastle.khanacademy.org/media/castles/Khan:master/exercises/probability_1.html?seed=184&problem=2&debug&activity=%5B%5D)

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