AP Statistics : Rules of Probability

Study concepts, example questions & explanations for AP Statistics

varsity tutors app store varsity tutors android store

Example Questions

Example Question #31 : Probability

Automobile license plates in a certain area display three letters followed by three digits, and the letters Z and N are not used. How many plates are possible if neither repetition of letters nor of numbers is allowed?

Possible Answers:

None of the other answers

Correct answer:

Explanation:

If no letters are to be repeated and the letters Z and N are not used, then there are  possible 3-letter combinations. If no digits are to be repeated, then there are  possible 3-digit combinations. Multiply these two results together, and you get a total of 8,743,680 possible license plates .

Example Question #11 : How To Use The Multiplication Rule

Tina and her 2 close friends are going to get together to see a movie Friday night, and Tina wants to determine the probability that they will all want to see the same one.  There are 6 movies playing in the local theater, so the friends each wrote down the name of the movie they want to see. (They are all appealing movies that and have an equal chance of being chosen.)

What is the probability that all 3 girls will choose the same movie out of the 6 playing?

 

 

Possible Answers:

Correct answer:

Explanation:

When determining the probability of independent events, you multiply the probability of each event occurring. 

The probability of a single girl choosing a specific movie is 1 out of 6, so to find the probability of this happening the same 3 times, you multiply 1/6 three times:

Example Question #32 : Probability

Mike's five-person family is going out to dinner, and each person is planning to order a soda. The restaurant offers Soda1 and Soda2 only, and each family member likes both sodas equally.

What is the probability that all 5 family members will order Soda1?

Possible Answers:

Correct answer:

Explanation:

When determining the probability of several events occuring independently, you use the multiplication rule, meaning that you multiply the probability of each individual event occuring. 

In this problem, the events are independent, meaning that each person's soda order does not affect the probabilty of someone else's order.

The probability of one person choosing Soda1 is 1 out of 2, or .

 

The probability of all five people ordering Soda1 is:

Example Question #34 : Statistical Patterns And Random Phenomena

Jenna and her two sisters are all picking a random number from 1 to 10.

What is the probability that they will all choose the same number?

Possible Answers:

Correct answer:

Explanation:

When determining the probability of several events occuring independently, you use the multiplication rule, meaning that you multiply the probability of each individual event occuring. 

 

In this problem, the events are independent, meaning that each person's number choice doesn't impact the other person's number choice

The probability of one person choosing a specific number is .

 

The probability of all 3 people choosing the same specific number is:

Example Question #12 : How To Use The Multiplication Rule

A person rolls a single 6 sided dice. What is the probability they will roll a 2 or a 4?

 

Possible Answers:

Correct answer:

Explanation:

In a single roll of a dice, rolling a 4 is mutually exclsuve of getting a 2. Therefore we will use the addition rule to find the probability of getting a 2 or a 4.

First, find the probability of each seperate event.

Because the problem asks for the probability of a 2 "or" a 4, we will add the probability of each event.

Example Question #11 : How To Use The Multiplication Rule

A tutoring agency helps match tutors with students. The agency knows that  percent of all the tutors it places with students will leave the position within a year, but after the first year, only  percent of the tutors who stay on will leave. At the start of the school year, an elementary school hires  tutors from the agency, then the next year it hires  more. How many of the  tutors are expected to still be working with their assigned students at the end of the second year? 

Possible Answers:

Correct answer:

Explanation:

If the school starts with  tutors, it can expect  to still be tutoring at the end of the first year. After that, we expect only  percent or  tutors to leave. So at the end of the second year, there will be the  original tutors hired the at the start of the first year and the  remaining tutors who were hired at the start of the second year. .

Example Question #33 : Probability

If you have a deck of cards, what is the probability that you draw a spade after you drew a non-spade on the first draw without replacement?

Possible Answers:

Correct answer:

Explanation:

You must use the multiplication rule which is the probability of one event happening after one has already taken place is the product of both probabilities.  The probability of drawing a non-spade on the first draw is .  Since there is no replacment, there are now 51 cards in the deck.  The probability of drawing the spade on the second draw is .  The probability of both happening after one another is then  = 

Example Question #31 : Probability

Charlene has 7 blue chips, and Karen has 5 green ones. They decide to mix them up in a bag, then Karen will pick 5 chips and Charlene gets the rest.

 

What is the likelyhood Karen ends up with 5 chips of the same color?

 

Possible Answers:

Correct answer:

Explanation:

There are 2 ways Karen gets 5 chips of the same color: she gets all 5 greens, or 5 blues. there is only 1 way to get all 5 greens: probability of which can be calculated as;

to calculate p(allblue) we use:

since order doesn't matter and chips are being drawn without replacement, this is an acceptable calculation and summing them yields 

Example Question #32 : Probability

A ninja is training for his super secret mission, throwing grappling hooks at targets ft away. He has determined over the last practice session, that he makes his shot roughly % of the time. What is the maximum number of shots the ninja will need to take to have a % chance of hooking his grapnel?

Possible Answers:

Correct answer:

Explanation:

Total likelihood of making the shot = sum of likelihood of making 1st shot+2nd shot+...

1st shot=

2nd shot=.

3rd shot=

4th=

5th=

6th=

Example Question #33 : Probability

Frank of the forest is a monkey man who was raised by chimpanzees from a very young age. He enjoys swinging with his monkey friends. When Frank jumps from one tree to another, the chance that he falls increases by 10% for every foot he jumps past his armspan of 3 feet. (he has a 10% chance of falling at 4ft)

One day, Frank watches his monkey friends do  jumps, one ft, one ft, another ft, one ft, a ft jump, another two ft jumps, and then a ft jump

Frank knows from experience not to take a jump route if the likelihood of him falling goes above %. At what point in this -jump route should Frank stop and rest?

Possible Answers:

Frank can finish this route, none of the jumps have above a % chance of falling

After  jumps, before the two ft jumps

After  jumps, before the ft jump

After  jumps, before the last ft jump

After  jumps, before the ft leap

Correct answer:

After  jumps, before the last ft jump

Explanation:

This is multiplication rule. let's calculate the chance frank doesn't fall.

For the first jump this is , the first two is , first 3: , first 4:, first 5:, first 6:, first 7:.

Since the 7th jump drops his survival rate below % (his fall rate above 60%) we would caution Frank to stop after his 6th jump, even if the 7th jump isn't that far

Learning Tools by Varsity Tutors