Q:

Number of Strings with two specific letters

How many ways can you construct a string four letters (from 26 alphabet characters) that have both the letters j and k in them?

A:

It is easier to count the complement.
There are $25^4$ strings that don't have $j$, and $25^4$ that don't have $k$. If we add these, we will count twice the strings that have neither $j$ nor $k$. There are $24^4$ of these. So the required number is
$$26^4 -\left(25^4+25^4-24^4\right).$$

