A random variable X is being measured for a representative sample of individuals from a population. If values of X for individuals in a population are known to vary by a standard deviation of ┠= 15, what is the minimum sample size required to ensure a 95% confidence interval for the true mean µ has a width of no more than 10?
(a) 9
(b) 10
(c) 34.57
(d) 35
(e) 138.30
(f) 139