You expect to use μ mangos each day. because of the variability you will actually sell more or less each day. to understand this, find out how many less or more than μ you expect to sell 80% of the time. that is, find two values equal distance from the mean such that 80% of all values fall between them. specifically find what number of x mangos where 80% of the values fall between μ-x and μ+x