Discussion 4

For the probability problems, please select one below to work on and share your computations and answer.

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Probability Puzzle 1: The Birthday Problem

There are 23 people in this class. What is the probability that at least 2 of the people in the class share the same birthday?

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Probability Puzzle 2: The Game Show Paradox

Let’s say you are a contestant on a game show. The host of the show presents you with a choice of three doors, which we will call doors 1, 2, and 3. You do not know what is behind each door, but you do know that behind two of the doors are beat up 1987 Hyundai Excels, and behind one of the doors is a brand new Cadillac Escalade. The cars were placed randomly behind the doors before the show, and the host knows which car is where. The way the game is played out is as follows. The host lets you choose a door. Assume you choose door #1. Before he opens door #1 to let you see what you have chosen, he opens one of the remaining doors, say door #3, to reveal a Hyundai Excel (he will always open one of the remaining doors that has the booby prize), and asks you whether or not you want to change your choice to door #2. What do you tell him?

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Probability Puzzle 3: Flipping Coins

If you flip a coin 3 times, the probability of getting any sequence is identical (1/8).

There are 8 possible sequences: HHH, HHT, HTH, HTT, THH, THT, TTH, TTT

Let’s make this situation a little more interesting. Suppose two players are playing each other. Each player choses a sequence, and then they start flipping a coin until they get one of the two sequences.

We have a long sequence that looks something like this: HHTTHTTHTHTTHHTHT…. We continue until one of the two wins.

Do you think this is a fair game, and that under these rules each sequence has an equal chance to appear first?

Think again! If you chose HHH and I chose THH, I have a much higher chance that you do!

The only way that you win is if the first three tosses are HHH. In any other event, I win.

Agree? Do you see why?

For the sequence HHH to appear anywhere except the first three flips, it must come after a T, right? So, the actual sequence for you to win is THHH.

But if there is a sequence of THHH then I already won before that sequence is over (because my sequence was THH).

So, THH will win 7 times out of 8. HHH will only win if the first three are HHH (a one in eight chance).

Suppose you are going to flip a coin until you get the sequence HTH. Say this takes you x flips. Then, suppose you are going to flip the coin until you get the sequence HTT. Say this takes you z flips. On average, how will x compare to z? Will it be bigger, smaller, or equal?

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Probability Puzzle 4: Disease Testing and False Positives

Assume that the test for some disease is 99% accurate. If somebody tests positive for that disease, is there a 99% chance that they have the disease?

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Probability Puzzle 5: A girl named Florida

Here’s a three part puzzler:
1. Your friend has two children. What is the probability that both are girls?
2. Your friend has two children. You know for a fact that at least one of them is a girl. What is the probability that the other one is a girl?
3. Your friend has two children. One is a girl named Florida. What is the probability that the other child is a girl?