Statistical Methods
posted by Mete .
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?

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.