if 5 cards are drawn from a deck of cards without replacement, what is
the probability that at least one of the cards is a heart?

I'd say it's 1-[Pr. that no hearts are drawn]. On card #1 you have
39/52 chance of a non-heart. C#2 (given C#1 a non-heart) = 38/51. C#3
(conditional that C31 and C#2 are non-hearts) = 37/50. C#4 = 36/49.
C#5 = 35/48. Multiply all these probabilities since they are
conditional, and you get ([39x38x37x36x35] divided by
[52x51x50x49x48]) = 22.15% that no cards are hearts. Therefore, the
probability that at least one card is a heart is 1 minus this number,
or 77.85%.