Obliczanie przedziału t dla średniej

An economist studying fuel costs wants to estimate the mean price of gasoline in her state on a certain day. On that day, she takes an of $12$‍ gas stations and finds the sample mean price (in dollars per gallon) is $\overline{x}=\mathrm{\$}2.29$‍ with a standard deviation of $s_x=\mathrm{\$}0.20$‍. The prices in the sample are roughly symmetric with no distinct outliers.