The expenses of customers follows a normal distribution with mean equals to $428, and standard deviation equals to $100. A customer spending $600 or more will receive a gift. If the probability of the shopping centre giving out gifts is at least 0.99, find the smallest no. of customers visiting the store. My solution is something like: X: expenses of customers X~N(428, 100) P(X>= 600) = P(Z>= (600-428)/100 ) = P(Z>= 1.72) = 0.5 - A(1.72) From the table, P(X>= 600) = 0.5 - 0.4573= 0.0427 n: no. of customers 0.0427^n >= 0.99 <----Why isnt this expression incorrect? There is another expression (1-0.0427)^n <= 0.01, which is correct. But as far as i'm concerned, they should be both correct Please help!