What sample size should be used to estimate the difference in the mean concentrations of rainfall impurities between the

[answerhappygod]

What sample size should be used to estimate the difference in
the mean
concentrations of rainfall impurities between the counties Cornwall
and Middlesex to
within 0.1 microgram of the true difference with a probability of
0.95. Assume that the
standard deviation for both counties is estimated to be 3.6
micrograms.