Enter that here --> Mean, Variance, and Standard Deviation
Now, since your sample size is less than 30, we should not do the normal test, but rather, the Student t-test. To see how that works, go here:
Student t-Test Confidence Interval for Mean
Using our mean and standard deviation that we calculated, n=20, and 95 for our confidence interval, I get:
PS - Scroll down in the math for a message on how to calculate this using Excel.
For your questions c and d, you can calculate a 99% confidence interval for your current sample size, but I'm not sure I understand what you mean by what sample size needed to ensure 99% confidence.