Question 293487
suppose a fair coin is tossed 7 times. What is the probability of flipping exactly 2 heads?
<pre>
That's a binomial problem.

{{{(nCx)p^x*q^(n-x)}}}

n = number of trials (tosses) = 7
x = number of successes required = 2
p = probability of one success in one trial (toss) = {{{1/2}}}
q = 1-p = 1 - {{{1/2}}} = {{{1/2}}}

{{{(7C2)(1/2)^2*(1/2)^(7-2)}}}
{{{(7C2)(1/2)^2(1/2)^5}}}
{{{((7*6)/(2*1))(1/2)^7}}}
{{{(42/2)(1/(2^7))}}}
{{{21/(2^7)}}}
{{{21/128}}}
{{{0.1640625}}}

Edwin</pre>