Statistics for Economics and business

1. Show the probability distribution function of the number of heads when three fair coins are tossed independently?

2. A student needs to know details of a class assignment that is due the next day and decides to call fellow class members for this information. She believes that for any particular call the probability of obtaining the necassary information is .4. She decides to continue calling class members until the infromation is obtained. Let the random variable X dentoe the number of calls needed to obtain the information?

a) Find the probability function of x.

b) Find the cumulative probability function of x.

c) Find the probability that at tleast three calls are required.



Any help would be appreciate... need to see work with problem though so I can understand... thanks to anyone who helps!

Here is #1.
\(\displaystyle \L P(X = k) = \left( {\begin{array}{c}
3 \\
k \\
\end{array}} \right)\left( {\frac{1}{2}} \right)^3 \quad ,k = 0,1,2,3\)