I'm in preparation for my final exam and I've just realised I don't know what formulas to use for what questions.

I would love help with this question.. or if you could point me in the right direction... thanks in advance

In the past, a particular chemical company produced a mean of 400 kgs of a certain type of plastic per day.

Now, using a newly developed and less expensive process, the sample mean daily yield of plastic for the first 50 days of production was 394.2 kgs with a sample standard deviation of 9.5 kg.

a) does the above sample information provide sufficient evidence to indicate that the true mean daily yeild of plastic for the new process differs significantly from that for the old procedure? Perform an appropriate statistical test at the 5% significance level, commenting on your choice of test.

b) construct a 95% confidence interval for the current true mean daily yeild of plastic.

c) the company has decided that it wishes to estimate the current true mean daily yield using the new process with more accuracy than that in b) above. if the allowable sampling error is reduced to 1.8 kg,how many days of production should now be included in the sample, assuming 95% confidence level? (you may take 9.5 kg as a reasonable approximation of the true standard deviation.)

Now, I know that the population mean is 400 kg, the sample mean is 394.2, n=50 and sample standard deviation is 9.5kg. I just don't know what to do with this information and what formulas to put it in.