I was watching the video here. For an $n$ object numeric list, consider $x_{i}$ to be the $n$th value along its index.
$\text{Percentile Rank of $x_i$}=\frac{\text{Number of Values Below } x_i}{n}\cdot 100$.
The number of values below $x_i$ is $i-1$. So, $\text{Percentile Rank of $x_i$}=\frac{i-1}{n}\cdot 100\implies \frac{\text{Percentile Rank of $x_i$}}{100}\cdot n+1=i$.
Why does this value say $i=\frac{percentile}{100}(n+1)$ in the video? Wouldn't it make sense to add one like the equation above instead of the $\frac{percentile}{100}$? I feel like I am missing something subtle.