We want to write the formula to evaluate the average for the second semester, which means we will use Feb, Mar, Apr, and May to evaluate it, they are
[tex]x_6,x_7,x_8,x_9[/tex]
Writing the formula:
[tex]\mu=\frac{x_6+x_7+x_8+x_9}{4}=\frac{\sum_{n\mathop{=}6}^9x_n}{4}[/tex]
Or
[tex]\mu=\frac{1}{4}\sum_{n\mathop{=}6}^9x_n[/tex]
Which is
[tex]\mu=\frac{1}{4}\sum_{n\mathop{=}6}^9x_n=\frac{80+78+78+67}{4}=\frac{303}{4}=\$75.75[/tex]
Rounding it to the nearest dollar:
[tex]\mu=\$76[/tex]
The average cable bill is $76
