A set of four results for the cadmium content of a water sample had a mean of $0\mathrm{.}35ppm$ and a standard deviation of $0\mathrm{.}004ppm$ If $t$ value at the $90\%$ confidence level is $2\mathrm{.}35$

Calculate,

RSD

Standard error of mean

The $90\%$ confidence interval for the mean is $0\mathrm{.}35+-$