is it the same as calculating P(X>=3)??
the probability of successes in trials is given by:
where is the probability of success, and is the probability of failure.
here, p = 0.4, and q = 0.6 and n = 15
so you want:
now, on my calculator, there is a button. so to find say, i would enter, "15, , 2 = "
but these are easy enough to work by hand if you're not sure about your calculator.