Posted by Mete on Tuesday, March 11, 2008 at 11:31pm.
Calculate the probability that 3 or fewer rainy days occur during the next 15 days. The probability any day is rainy is .2.
For this question I tried using binomial distribution by setting x = 0, 1, 2, 3. p=.2 and n =15. After doing all of them, Do add the probabilities to get the answer?

Statistical Methods  PsyDAG, Wednesday, March 12, 2008 at 2:13am
To get the probability that all events would occur, you need to multiply the probability of each of the individual events.
The probablitiy of no rainy days is .8^15.
The probability of one rainy day is .2 * .8^14.
The probability of two rainy days is .2^2 * .8^13.
The probability of three rainy days is .2^3 * .8^12.
The probability of being either one or the other of these four probabilities is found by adding the four products above.
I hope this helps. Thanks for asking.