Confidence Intervals Help!!

How do I do this qn? I have the formula and stuff I just cant get it all right.

In a sample of 85 teenagers, 14 were classified as Vitamin C deficient.
(a) Find a 95% confidence interval for the true proportion of teenagers with vitamin C defi-
(b) Is it reasonable to claim that 20% of teenagers will be deficient in vitamin C? Using the
result in a), explain your answer.
(c) What sample size is needed to estimate the true proportion with 95% confidence and a
margin of error of no more than 0.05? Use the original sample data as your estimate for