Here’s an example:
17.A jar contains only red marbles and white marbles. If the probability of selecting a red
marble is , which of the following expressions gives the probability of selecting awhite marble in terms of r and y ?A)B)
C)
D)
Here’s How to Crack It
Plug in! You could make the probability of choosing a red marble be . Then = , which means that r =
2 and y = 3.
Now, to get the numerical answer, you need to remember that the probabilities of all the things that can
happen always add up to 1. Since there are only red marbles and white marbles in the jar, choosing a red
marble or choosing a white marble are the only things that can happen. So, the probability of selecting a
white marble can be found by subtracting the probability of getting a red marble from 1: Probability of
white = 1 – = . That’s your target, so circle it.
Now, go find the answer that gives you when r = 2 and y = 3. Choice (B) is . Since
no other answer works out to , (B) is the answer.
Finally, let’s look at a probability question based on a chart. Again, getting to the correct answer involves
reading the chart carefully to find to right numbers to use.