Find Sample Size for a Given Margin of Error- Please Help

Question

A researcher studying the lifespan of a certain species of bacteria. A preliminary sample of 30 bacteria reveals a sample mean of  hours with a standard deviation of  hours. He would like to estimate the mean lifespan for this species of bacteria to within a margin of error of 0.5 hours at a 98% level of confidence.What sample size should you gather to achieve a 0.5 hour margin of error?He would need to sample  ______  bacteria.

Answer 1

sample size = (z-critical * standard deviation/margin of error), where z-critical is z-score corresponding to P(z < 0.99).

= (2.33 * 5/0.5)^2 =~ 543