Please state the formulas

Exercise 3.1 You have been employed by an airline and given the task of estimating the per- centage of flights that arrive on time, i.e. no later than 20 minutes after the scheduled arrival time. How many flights should you survey in order to be 90% confident that your estimate is within four percentage points of the true population percentage? Solve this problem under each of the following assumptions: a) nothing is known about the percentage of on-time flights of your airline; b) for a previous year, 85% of all flights of your airline were on time (based on an official statistic from the government). Also state the formulas that you are using