Answer
The total number of new customers attracted during the first 5 days is 800.
Work Step by Step
For this arithmetic sequence, we have $a_{1} = 80$ and $d = 40$, the general term $a_{n} = 80 + (n - 1)(40)$ where n is the number of days.
The total number of new customers attracted during the first 5 days is
$\sum\limits_{n = 1}^5 a_{n} = \sum\limits_{n = 1}^5 [80 + (n - 1)(40)]$
= $\frac{5(80 + a_{5})}{2}$
= $\frac{5(80 + 240)}{2}$ ($\because$ $a_{5}$ = 80+(5-1)(40) = 240)
= 800 new customers
