How Likely Is That?
Page 3

Shelly Berman shelly@mathforum.org


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Probability Experiments

Experiment 1

If you toss a single number cube (one die), what is the probability of rolling an even number?

One way of thinking about probability is to make a fraction that shows:

(the number of things you want)

(the number of possibilities)
.

  • How many different numbers could you get when you roll a number cube?
  • How many of these are even?

When you roll a number cube you can get any of the six numbers: 1, 2, 3, 4, 5, or 6.

Three of these numbers are even: 2, 4, and 6.

That is, the probability of rolling an even number can be written as: P(even number) = (3 numbers we want)/(6 possible numbers) = (3/6).

So, the probability of rolling an even number on a number cube is (3/6).

  • Where is this on the probability scale from 0 to 1? How do you know?

Experiment 2

How do you think we could measure the probability of tossing a die and getting a 1?

  • Is it more likely that you will get a 1 or that you won't get a 1?
  • In fact, can you measure the probability of tossing a die and getting any number but a 1?
Experiment 3

Here's a set of twenty cards. Each card has a number from 1 to 10, and the number is either red or black.

If I draw one card without looking, what is the probability that the card I draw will:
  • be red?

  • show an even number?

  • show a number less than 3?

  • show a black 7?
Can you think another outcome from drawing one card that has a probability of:
  • (1/2)?

  • (1/4)?

  • (4/20)?

  • (1/20)?

  • (19/20)?

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