Coffee and donuts at a certain coffer shop all the customer

Coffee and donuts - at a certain coffer shop, all the customers buy a cup of coffee some also buy a doughnut. The shop owner believes that the number of cups he sells each day is normally distributed with a mean of 320 cups and a standard deviation of 20 cups. He also believes that the number of doughnuts he sells each day is independent of the coffee sales and is normally distributed with a mean of 150 doughnuts and a standard deviation of 12. (Exercise 59 from the book) The shop is open every day but Sunday. Assuming day to day sales are independent, what\'s the probability he\'ll sell more than 2000 cups of coffee a week? Ohm= E(X) = E (Cups sold in a week) alpha = SD (Cups sold in a week) = Calculate Z-Score

Solution

Normal Distribution
Mean ( u ) =320
Standard Deviation ( sd )=20
Normal Distribution = Z= X- u / sd ~ N(0,1)                  
Selling more than 2000of coffee a week except sunday is = 2000/6 = 333.33
P(X > 333.33) = (333.33-320)/20
= 13.33/20 = 0.6665
= P ( Z >0.666) From Standard Normal Table
= 0.2525