Question 697006

PLEASE i need your help

ok im going to try to put it up that way again

Consider the accompanying contingency table, which presents the results of an advertising survey about the use of credit by Martan Oil Company customers. 
Number of Purchases at Gasoline Station Last Year
Preferred Method of Payment   0-4     5-9   10-14  15-19  >20  sum
Cash                            150     100    25    0      0    275
Oil-Company Card             50       35    115    80   70    350                
National or Bank Credit Card 50       60    65     45    5   225
Sum                          250      195   205    125  75    850
 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 oil-company credit card? 
 d. How many customers made 20 or more purchases last year? 
i tried to put a cell in here and it wouldnt let me  so this is the best i could do  thanks


a)	850
b)	Because there are 850 customers (independent variable), and their individual method of payment (dependent variable). 
c)	350
d)	75