To explain easier, I'll put it into a problem format.
Assume the population is a deck of $60$ cards ($N = 60$).
Of these cards, $35$ are blank and $25$ have letters on them.
The quantity of cards with letters are known to be as follows:
$A=4$;
$B=3$;
$C=3$;
$D=2$;
$E=2$;
$F=2$;
$G=2$;
$H=2$;
$I=2$;
$J=2$;
$K=1$.
How would one go about calculating the odds of drawing at least $5$ unique cards with letters on them in a sample of $15$ cards ($n=15$)? While it is easier to get an answer by modelling with code, how would one calculate it mathematically?