Consider the accompanying contingency table, which presents the results of an advertising survey about the use of credit by Martan Oil Company customers.

a. How many customers were surveyed?
b. Why are these bivariate data? What type of variable is each one?
c. How many customers preferred to use an oilcompany card?
d. How many customers made 20 or more purchases last year?
e. How many customers preferred to use an oilcompany card and made between five and nine purchases last year?
f. What does the 80 in the fourth cell in the second row mean?

Number of Purchases at Gasoline Station Last Year Preferred Method of Payment 0-4 5-9 10-14 15-19 20 Sum Cash Oil-Company Card Bank Credit Card 0 0 275 50 35 115 0 70 350 50 60 65 45 5 225 Sum 250 195 205 125 75 850