about t-statistics good evening every body 
I have a question about t-statistics
in first place I need to use t-statistics to estimate the population mean from a sample set, as you see I have a data set the mean and standard deviation are known for sample but not for the population.
how can I use t-statistics to determine the population mean I have X bar and S and n values for sample  n=45
how can I compute the degree of freedom and how can I determine the confidence level?
thanks for your time 
huge regards 

$df=n-1$
You need to set the confidence level yourself. $\alpha =.05$ is standard.
Confidence interval:
$$Upper limit=Xbar+\frac{tquantile(1-\alpha/2,df)*S}{\sqrt{df}}$$
$$Lower limit=Xbar-\frac{tquantile(1-\alpha/2,df)*S}{\sqrt{df}}$$
Where tquantile is the quantile from a t-distribution chart or obtained from statistical software. It is 2.015368 so you have:
$$Upper limit=Xbar+\frac{2.015368*S}{\sqrt{df}}$$
$$Lower limit=Xbar-\frac{2.015368*S}{\sqrt{df}}$$
In R:
> qt(.975,44)
[1] 2.015368

Note: I'm not sure if you need to divide by $\sqrt df$, depends on whether S is the standard dev of the sample or the individual observations...
