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
[tex]\begin{gathered} N=89 \\ T=4 \\ x=\frac{89}{4} \\ x=22.25 \end{gathered}[/tex]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
[tex]\begin{gathered} y=60 \\ z=123 \\ w=\frac{60}{4}+\frac{123}{4} \\ w=\frac{60\times4+4\times123}{16} \\ w=45.75 \end{gathered}[/tex]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
[tex]\begin{gathered} b=\frac{a+N+y+z}{4} \\ b=\frac{126+89+60+123}{4} \\ b=\frac{398}{4} \\ b=99.5 \end{gathered}[/tex]