Jerry Crawford – June 15, 2011
Thinking Ahead about Probability 1 – Exercise Key
Exercises
1. A six-sided die is rolled. What is the probability of getting a number less than 3?
2 1
The probability of rolling a number less than 3 is P(1) + P(2) =  .
6 3
2. A six-sided die is rolled. What is the probability of not rolling a 5?
The probability of rolling any other number except 5 is equal to: 1 – P(5) =
5
.
6
3. What is the probability of choosing a vowel by choosing one letter randomly from our 26letter alphabet? (Do not count y as a vowel.)
5
There are 5 vowels out of 26 possible outcomes, so P(vowel) =
.
26
4. For choosing one pair randomly from the sock drawer above, what is the probability of not
choosing a blue pair or a red pair?
The probability of not choosing a blue pair or a red pair is 1 – P(blue) – P(red) = P(black) +
P(white) = 40%.
5. What is the probability of choosing the letter “s” by randomly choosing a letter from the word
Mississippi? What is the sample space?
4
There are 4 “s” out of 11 letters, so P(s) =
. The sample space is M,i,s,p.
11
6. A card is selected randomly from an ordinary deck of 52 cards. What is the probability of
selecting an 8 or a black king or a red 10?
There are four eights, two black kings, and 2 red tens out of 52 cards, so:
8
2
 .
P(8 or black king or red 10) =
52 13
7. How many possible outcomes are there for two dice rolled at the same time?
There are 62  36 possible outcomes.
8. What is the probability of obtaining the sum of 5 for two dice rolled at the same time?
There are four outcomes that sum to five: 1 + 4, 4 + 1, 2 + 3, and 3 + 2.
4 1

Thus, P(5) =
36 9