Khan/khan-exercises — Issue #12042: Probability 1 - Probability is wrong!
Author: KhanBugz
Created: 2012-01-25 00:37:03+00:00
State: open
Labels: ie8, ie9, vista, wrong-or-unclear

It said:

A fair coin is flipped three times. What is the probability of getting heads at least twice? Write your answer as a simplified fraction.

There are 2⋅2⋅2=2 3 =8 possibilities for the sequence of flips.

The possibilities are HHH, HHT, HTH, HTT, THH, THT, TTH, and TTT.

There are 4 favorable outcomes: HHH, HHT, HTH, and THH.

The probability is 4/8 =1/2 .

I say

1/2 is not the answer, a 50 percent chance would be if they were asking for the coin being flipped 4 times. In my opinion the answer is 2/3. That'a just me, I could be wrong.

Thanks!

[probability_1.html?seed=186&problem=2](http://sandcastle.khanacademy.org/media/castles/Khan:master/exercises/probability_1.html?seed=186&problem=2&debug)

[Answer timeline](http://sandcastle.khanacademy.org/media/castles/Khan:master/exercises/probability_1.html?seed=186&problem=2&debug&activity=%5B%5B%22incorrect-activity%22%2C%22%5C%222%2F3%5C%22%22%2C105%5D%2C%5B%22hint-activity%22%2C%220%22%2C2%5D%2C%5B%22hint-activity%22%2C%220%22%2C2%5D%2C%5B%22hint-activity%22%2C%220%22%2C2%5D%2C%5B%22hint-activity%22%2C%220%22%2C3%5D%5D)

["2/3"]

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