THE NEXT QUESTIONS ARE BASED ON THE FOLLOWING INFORMATION:
You are the webmaster for your firm's web-site.From your records,you know that the probability that a visitor will buy something from your firm is 0.23.In one day,the number of visitors is 952.
-What is the probability that at least 210 of them will buy something from your firm? Use the normal approximation to the binomial.
A)0.7549
B)0.2549
C)0.2451
D)0.7451

Question

Quizplus · Accepted Answer

We can use the normal approximation to the binomial by finding the mean and standard deviation of the distribution. 
Mean = np = 952 * 0.23 = 218.96 
Standard Deviation = sqrt(np(1-p)) = sqrt(952*0.23*0.77) = 12.08 
We want to find P(X >= 210), which is equivalent to P(Z >= (210-218.96)/12.08) = P(Z >= -0.74) 
Using a standard normal distribution table or calculator, we find this to be 0.7549.