Khan/khan-exercises — Issue #15721: Probability 1 - Ratio is wrong
Author: KhanBugz
Created: 2012-03-08 16:16:13+00:00
State: open
Labels: firefox, win7, wrong-or-unclear

(I asked for hints because I didn't understand how it was wrong)

"A fair coin is flipped three times. What is the probability of getting heads exactly 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 3 favorable outcomes: HHT, HTH, and THH.

The probability is 3/8."

If I'm not mistaken, there are 4.

HHH, HHT, THH, and HTH.

[probability_1.html?seed=110&problem=2](http://sandcastle.khanacademy.org/media/castles/Khan:master/exercises/probability_1.html?seed=110&problem=2&debug)

[Answer timeline](http://sandcastle.khanacademy.org/media/castles/Khan:master/exercises/probability_1.html?seed=110&problem=2&debug&activity=%5B%5B%22incorrect-activity%22%2C%22%5C%224%2F9%5C%22%22%2C539%5D%2C%5B%22hint-activity%22%2C%220%22%2C4%5D%2C%5B%22hint-activity%22%2C%220%22%2C4%5D%2C%5B%22incorrect-activity%22%2C%22%5C%224%2F8%5C%22%22%2C21%5D%2C%5B%22hint-activity%22%2C%220%22%2C3%5D%2C%5B%22hint-activity%22%2C%220%22%2C3%5D%2C%5B%22correct-activity%22%2C%22%5C%223%2F8%5C%22%22%2C61%5D%5D)

["4/9","4/8","3/8"]

Mozilla/5.0 (Windows NT 6.1; WOW64; rv:10.0.2) Gecko/20100101 Firefox/10.0.2

MathJax is loaded, ready, queue length: 0