In an intra-squad swim competition, men's freestyle 100 swim times at a certain university ranged from 43.89

Question:

In an intra-squad swim competition, men's freestyle 100 swim times at a certain university ranged from 43.89 seconds to 51.96 seconds.
(a) Estimate the standard deviation using Method 3 (the Empirical Rule for a normal distribution).
(b) What sample size is needed to estimate the mean for all swimmers with 95 percent confidence and an error of ±0.50 second?