probability question from outline

jordan

Can someone give me the computation for "What is the probability that a card is a heart, given that it's red?"

mark

If the card is red, then it's been narrowed down to one of two suits - hearts or diamonds and those have 13 cards each so the probability oughtta be $13/26 = 0.5$.

That's in contrast to the situation where you're not given that the card is red. Then, the probability that the card is a heart is $13/52 = 0.25$.