# Probability-Math

posted by
**Alia**
.

Please forgive me for posting this for the 3rd time but I really need to learn this:

A researcher conducted a study of the access speed of 45 hard drives and concluded that his maximum error of estimate was 28. If he were to conduct a second study to reduce the maximum error of estimate to 7, about how many hard drives should he include in his new sample?

Select one:

a. 720

b. 45

c. 90

d. 180

Please can someone explain me how to do the above problem.

My working: 28/7 = 4, so 4^2=16

16 x 45 =720.

I am not sure if the above working is correct but please i really need help in learning this problem (question). So, Please can anyone assist me by showing the correct method for this.

Thanks.