How can I solve this?

A random sample of medical files is used to estimate the proportion p of all people who have blood type B.

If you have no preliminary estimate for p, how many medical files should you include in a random sample in order to be 85% sure that the point estimate p̂ will be within a distance of 0.04 from p? (Round your answer up to the nearest whole number.)

Answer part (a) if you use the preliminary estimate that about 10 out of 90 people have blood type B.
X medical files