How many $k$-substrings does an $n$-string have? (Consider identical $k$-substrings at different positions to be different.) How many substrings does and $n$-string have in total?

There are $S_k = n - k + 1$ possible substrings of length $k$ (one starting at the first position, one and the second, etc.)

In total there are:

$$ \begin{align} S &= S_1 + S_2 + \ldots + S_n \\ &= \sum_{i=1}^{n}S_i \\ &= \sum_{i=1}^{n}(n - i + 1) \\ &= \sum_{i=1}^{n}n - \sum_{i=1}^{n}i + \sum_{i=1}^{n}1 \\ &= n^2 - n(n + 1)/2 + n \\ &= n^2 - n^2/2 - n/2 + n \\ &= n(n + 1)/2 \qquad \text{(Duh!)} \end{align}$$

Obvious proof is obvious.