Answer
$35$ balls
Work Step by Step
The stack has a total of
$ a_{1}+a_{2}+a_{3}+a_{4}+a_{5}\qquad$ balls.
$a_{1}=\displaystyle \frac{1(1+1)}{2}=1$
$a_{2}=\displaystyle \frac{2(2+1)}{2}=3$
$a_{3}=\displaystyle \frac{3(3+1)}{2}=6$
$a_{4}=\displaystyle \frac{4(4+1)}{2}=10$
$a_{5}=\displaystyle \frac{5(5+1)}{2}=15$
Total = $1+3+6+10+15=35$
