What two numbers do you divide to get the probability?

Mr. McGee is the manager of the washer and dryer department of an appliance store. He wants to estimate the probability that a customer who comes to his department will buy a washer or dryer. He collected the following data during one week: 177 customers came to his department, 88 purchased washers, 64 purchased dryers, and 69 did not make a purchase.
Make a two-way table.
What to numbers do I divide to get the probability that a customer will buy a washer or dryer? I just need to know the numbers that I divide and I can do the rest.

1 Answer
Apr 15, 2018

#108 / 177#


Note that #88 + 64 + 69 = 221 > 177#, so clearly some people bought both a washer and a dryer. To figure out how to get the probability, we first need to figure out how many people made a purchase.

This is fairly straightforward, since we know how many did not make a purchase. If 69 people didn't make a purchase, then #177 - 69 = 108# people did make a purchase.

So our probability is the number of people who made a purchase divided by the total number of people. That is, #108 / 177#.