A probability problem is a ratio problem in disguise. When a problem asks what
the chance is that a particular thing will happen, all it’s really asking you to do is
to set up a ratio like this:
Here’s an example:
If there are 12 pairs of boxer shorts and 36 pairs of briefs in a huge laundry
bag, what is the probability that, at random, Bill will grab the boxer shorts?
Our “particular thing” here is grabbing a pair of boxer shorts, of which there
are 12. “Any of the things” is the number of underwear total, which is 48.
Bill has a 25% chance of grabbing a pair of boxers. Good luck, Bill!
On most probability problems, however, the SAT is going to ask for the
probability of multiple events.
A deck of cards includes 10 blue cards, 10 green cards, and 10 black cards.
What are the odds of drawing two black cards in a row, if no card is replaced
once it is drawn?
We see that we are dealing with two events (drawing one card, then drawing
another). The first event is easy.
The odds of our first event are ⅓. But our second event is trickier. Remember, if