statistics applied to flow

Hi there,
I have been trying to solve the problem below, but have had difficulties in solving it. I would really appreciate it if anyone could explain to me how to solve it.

The standard deviation of the flow is s=0.3 occuring where the mean concentration is 0.9. The mean concentration is measured ±3% with 95% confidence. Standard error given by e=2(s^2/N)^1/2 for 95% confidence. If instantaneous concentrations is ±4% with 95% confidence then how many independent measurements N are required?

Many thanks in advance:).