Use z-scores:
z = (x - mean)/sd
With your data:
z = (20 - 30)/9 = ?
Finish the calculation. Next, look at a z-table to determine your probability. (Remember that the problem is asking "$20 or more" when looking at the table.)
I'll let you take it from here.
A popular retail store knows that the purchase amounts by its customers is a random variable that follows a normal distribution with a mean of $30 and a standard deviation of $9.
What is the probability that a randomly selected customer will spend $20 or more at this store?
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