Find the minimum sample size required to estimate the unknow

Find the minimum sample size required to estimate the unknown population mean weight of women in one age group. We want 95% confidence that the sample mean is within 3.4 lb of the population mean, and the population standard deviation is known to be 25lb.

Solution

popltn std. dev. =25 lb ;
z= 1.959964 at 95% confidence
margin of error for sample mean = 3.4 lb

so, minimum sample size = (z* popltn s.d/margin of error)^2 = (1.959964*25 /3.4)^2 =207.69 =208(rounded)
so,the minimum sample size= 208