How Likely Is That?
Page 2

Shelly Berman shelly@mathforum.org

Page 1 || Page 2 || Page 3 || Page 4

Some things might happen or might not happen. For example, on any given day in Philadelphia, it might rain, sleet or snow, or it might not do any of these.

• What do you think the probability is that it will rain sleet or snow today?
  What makes you think that?

Some things are just as likely to happen as they are to not happen. For example, when I flip a normal coin, it is just as likely to land heads up as tails up.

• What do you think the probability is of something that is just as likely to happen as to not happen?

We would say that the probability of flipping a coin and getting a head is (1/2).

Where would you place that probability on the scale:

• What do you think the probability is of flipping a coin and getting a tail?

• Can you think of any situations besides a coin flip that has two outcomes that are just as likely to happen? We say these outcomes are equally likely.

• Can you think of situations where one outcome is more likely to happen than not happen?
  What would the probability be of the outcome that is more likely to happen?
  What makes you think that?

• Can you make a guess about the probability of the outcome that is less likely to happen?