Question

Michelle draws a card from a standard deck of 52 cards. She replaces the card and draws a second card. What is the probability that she draws a heart and then a spade? [Note: There are 13 each of hearts, spades, clubs, and diamonds.]

Answer 1

To draw a heart, one would be choosing 1 card of 13 possible hearts, and 0 from the remaining 39 non-hearts. With respect to the entire deck, one would be choosing 1 card from 52 total cards. So the probability of drawing a heart is

$\dfrac{\dbinom{13}1\cdot\dbinom{39}0}{\dbinom{52}1}=\dfrac{13\cdot1}{52}=\dfrac14$

When Michelle replaces the card, the deck returns the normal, so the probability of drawing any card from a given suit is the same, $\dfrac14$. In other words, drawing a spade is independent of having drawn the heart first.

So the probability of drawing a heart, replacing it, then drawing a spade is $\dfrac14\cdot\dfrac14=\dfrac1{16}$.