A firm plans research to find out the the mean (time) of which employees in senior level management surf the internet. A preliminary test indicates a mean time/wk of twelve hrs, a std dev. = 3 hrs. It is desirable to est. a mean internet use time within 1/4 hr. a ninety-five% conf level is assumed to be used. Find how many upper level managers need to be examined in a survey:

n ≥ (z*σ/E)^2

n ≥ (1.96*3/0.25)^2

n ≥ 553.2

n = 554