> My response > > I answered, strictly, what the OP asked: to use the > normal approximation to evaluate a binomial problem, > NOT what where the different procedures to do it, or > what was the best one. > The formula to find APPROXIMATELLY p(X<=a) is > > ______ z = (a + 0.5 - p*N) / s > ______ s^2 = p * (1-p) * N > > and p(X<=a) = F(z) , the Normal Standard DF at the > point z, X~ Binomial (p, N), p the constant > probability of success at each Bernouilli N trials. > This method is called currently the CLASSICAL > CONTINUITY APPROXIMATION, a standard method that was > indicated yet in this thread. > Jack Tomsky did not take into account. HE DIDN`T PAY > ATTENTION (!). > > Licas
The question did not ask for a normal approximation. It asked only for the probability. The professor would have liked my answer better than yours. Here is the original question.
"Past records show that at a given college 20% of the students who began as psychology majors either changed their major or dropped out the school. An incoming class has 110 beginning psychology majors. What is the probability that as many as 30 of these students leave the psychology program?"