If you flip a fair coin four times, what is the probability that there you get heads twice?
You can attack this problem in several ways.
Since there are only 16 combinations (2 for the first, times the second, third and fourth) you can write them out:
HHHH, HHHT, HHTH, HHTT, etc.
And filter out the right ones. Divide this by
You can use the binomial function on your GC.
I use a TI-83 Plus:
2nd-distr -- binompdf(4, 0.5, 2)
(This means: 4 tries, chance =0.5, exactly 2 good outcomes)