Answer
$108$ cards.
Work Step by Step
Given the rule for the number of cards in the n-th row in example 6,
$a_{n}=3n$,
The formula for the sum of a finite series is $S_{n}=n\displaystyle \cdot\frac{a_{1}+a_{n}}{2}$.
We first find $a_{1}$ and $a_{8}$.
$a_{1}=3\cdot 1=3$
$a_{8}=3\cdot 8=24.$
Substitute these values into the formula for $S_{8}$.
$S_{8}=8\displaystyle \cdot\frac{3+24}{2}=4\cdot 27=108$
