How to calculate confidence intervals?

Hi, I have a problem solving a task, I was hoping that someone could help me.

A machine fills bags with content. From the production a sample takes out and each bag is weighed.
In Table 1 is the result from the measurements. From previous experiences you know that the
weights follow a normal distribution.
Table 1: The weight on the bags in gram
100.7 95.9 103.8 101.2 100.5
102.1 101.0 96.2 99.4 99.4

a) Calculate a 95% confidence interval for the mean weight on the bags. What conclusions can one make from the confidence interval about the weight?

b) Which confidence level gives a width on the confidence interval of 5 gram?

c) How large sample size do you need to have if confidence interval width should be 5 gram and the confidence level is 95%. Assume that the estimated standard deviation is the same as in a).
Sorry forgot to mention that i started by adding all the values together and divided them by 10, got the number 100,02. then i calculated the S^2 and got the value 5,99. I then took 100,02-1,96*5,99 and 100,02+1,96*5,99 and got the answers 88,2796 and 111,7604.
Is it right so far? I have no idea how to go on from here.