# Category Archives: Counting

## Outcome space

Let $$\Omega$$ be an outcome space with 16 outcomes. $$A$$ and $$B$$ are events inside of $$\Omega$$. Event $$A$$ has 10 outcomes and event $$B$$ has 10 outcomes.

1. Determine all the possible values of $$\# (A\cap B).$$
2. Determine all the possible values of $$\# (A\cup B).$$
3. Determine all the possible values of $$\#(A^c\cup B^c).$$
4. Determine all the possible values of $$\# (A^c\cap B^c).$$

You roll a fair 6-sided die 3 times. What is the likelihood of getting exactly one 4, exactly one 5, or exactly one 6?

## Repeated Quiz Questions

Each week you get multiple attempts to take a two-question quiz. For each attempt, two questions are pulled at random from a bank of 100 questions. For a single attempt, the two questions are distinct.

1. If you attempt the quiz 5 times, what is the probability that within those 5 attempts, you’ve seen at least one question two or more times?
2. How many times do you need to attempt the quiz to have a greater than 50% chance of seeing at least one question two or more times?

## Which deck is rigged ?

Two decks of cards are sitting on a table. One deck is a standard deck of 52 cards. The other deck (called the rigged deck)  also has 52 cards but has had 4 of the 13 Harts replaced by Diamonds. (Recall that a standard deck has 4 suits: Diamonds, Harts, Spades, and Clubs. normal there are 13 of each suit.)

1. What is the probability one chooses 4 cards from the rigged deck and gets exactly 2 diamonds and no hearts?
2. What is the probability one chooses 4 cards from the standard deck and gets exactly 2 diamonds and no hearts?
3. You randomly chose one of the decks and draw 4 cards. You obtain exactly 2 diamonds and no hearts.
1. What is the probability you chose the cards from the rigged deck?
2. What is the probability you chose the cards from the standard deck?
3. If you had to guess which deck was used, which would you guess? The standard or the rigged ?

## Match play golf problem

This problem is motivated by the new format for the PGA match play tournament. The 64 golfers are divided into 16 pools of four players. On the first three days each golfer plays one 18 hole match against the other three in his pool. After 18 holes the game continues until there is a winner. At the end of these three days the possible records of the golfers in a pool could be: (a) 3-0, 2-1, 1-2, 0-3; (b) 3-0, 1-2, 1-2, 1-2; (c) 2-1, 2-1, 1-2, 1-2; (d) 2-1, 2-1, 2-1, 0-3. What are the probabilities for each of these four possibilities?

## Meeting in a Tournament

A tennis tournament is organized for $$2^n$$ players where each round is single elimination with $$n$$ rounds. Two players are chosen at random.

1. What is the chance that they meet in the first round or second round ?
2. What is the chance they meet in the final or semi-final ?
3. What is the chance they do not meet at all ?

[Sudov and Kelbert, p4 problem 1.2]