18.
Let:
x = Number of calendars sold per week by 3rd period class
N = Total number of calendars sold by 3rd period class
T = Number of weeks
19.
Let:
y = Number of calendars sold per week by 1st period class
z = Number of calendars sold per week by 2nd period class
w = Number of calendars sold per week by 1st and 2nd period classes
20.
Let:
a = Number of calendars sold per week by 4th period class
b = Average number of calendars sold in a week by all of her classes