# PSAT Math : How to find the probability of an outcome

## Example Questions

### Example Question #1 : How To Find The Probability Of An Outcome

If given two dice, what is the probability that the sum of the two numbers rolled will equal 9?

1/6

1/24

1/18

1/9

1/36

1/9

Explanation:

There are 36 possible outcomes of the additive dice roll. The way to roll a sum of 9 is 6 (and vice versa) and 3 or 5 and 4 (and vice versa). This is possible 4 of the 36 times, giving a probability the sum of the two dice rolled of 4/36 or 1/9.

### Example Question #1 : How To Find The Probability Of An Outcome

A bag contains 6 green marbles, 5 blue, and 9 red. What is the probability that you will select two green marbles from the bag?

6/20

3/38

5/42

9/100

21/190

3/38

Explanation:

There are 20 total marbles. Selecting the first green marble has a 6/20 chance, the second green marble has a 5/19 chance. This gives a total chance of 30/380, or a 3/38 chance.

### Example Question #1 : How To Find The Probability Of An Outcome

There is a special contest held at a high school where the winner will receive a prize of \$100. 300 seniors, 200 juniors, 200 sophomores, and 100 freshmen enter the contest.  Each senior places their name in the hat 5 times, juniors 3 times, and sophmores and freshmen each only once. What is the probability that a junior's name will be chosen?

1/24

1/6

5/8

2/5

1/4

1/4

Explanation:

The first thing to do here is find the total number of students in the contest. Seniors = 300 * 5 = 1500, Juniors = 200 * 3 = 600, Sophomores = 200, and Freshmen = 100. So adding all these up you get a total of 2400 names in the hat. Out of these 2400 names, 600 of them are Juniors. So the probability of choosing a Junior's name is 600/2400 = 1/4.

### Example Question #11 : Probability

Michelle is randomly drawing cards from a deck of of 52 cards. What is the chance she will draw a black queen followed by a 5 of any color, without replacing the cards?

2/663

2/169

4/169

2/2652

8/663

2/663

Explanation:

There are 2 black queens in the deck, one of spades and one of clubs, so there is a 2/52 chance a black Queen will be drawn and 4/51 chance of drawing a 5 of any color, since the queen has already been removed from the deck. Thus:  2/52 * 4/51 = 8/2652 → 2/663.

1

### Example Question #11 : How To Find The Probability Of An Outcome

Zack has 10 green, 14 red, 2 blue, and 6 black marbles in a bag. What is the probability that Zack will not randomly pick a red or blue marble from the bag?

5/16

1/2

15/32

1/3

3/16

1/2

Explanation:

To NOT choose a red or blue, leaves 6 black and 10 green to choose from. That leaves 16 marbles out of a total of 32 marbles, or a 1/2 chance.

### Example Question #11 : Probability

A bag contains four blue marbles, four red marbles, and two green marbles in a bag. If one marble is drawn, and then a second is drawn without replacement, what is the probability that at least one of the two marbles will be red?

2/5
26/45
2/3
4/9
11/15
Explanation:

We have blue, red, and green marbles in a bag. We need to consider all of the ways that we could draw at least one red. This means we can draw a red the first time, a red the second, or a red both times. These are the possible ways we could at least draw one red marble:

We can draw a red and then a blue.

We can draw a red and then a green.

We can draw a blue and then a red.

We can draw a green and then a red.

We could draw a red and then another red.

So we need to find the probability of each of these five scenarios. Then, we need to add these probabilities.

Let's look at the probability of the first scenario (red, then blue). The probability of drawing a red on the first time would be 4/10, because there are 10 marbles, and four are red. On the second draw, we don't put this marble back. This means we only have 9 marbles in the bag, and four of them are blue. Thus, the probability of the second draw being blue would be 4/9.

The probability of drawing a red and then a blue is equal to the product of these two events. Whenever we want to find the probability of one event AND another, we need to multiply. Thus, the probability of drawing the red AND then a blue would be (4/10)(4/9) = 16/90.

We can calculate the probability of the other four possibilities in a similar fashion.

The probability of drawing a red and then a green is (4/10)(2/9) = 8/90

The probability of drawing a blue then a red is (4/10)(4/9) = 16/90

The probability of drawing a green then a red is (2/10)(4/9) = 8/90

The probabilty of drawing a red then a red is (4/10)(3/9) = 12/90

To find the total probabilty, we need to add up the probabilities of the five different scenarios. Whenever we want to find the probability of one event OR another, we add. So the final probablity is

16/90 + 8/90 + 16/90 + 8/90 + 12/90 = 60/90 = 2/3.

The probability of drawing at least one red is 2/3.

### Example Question #11 : How To Find The Probability Of An Outcome

There are 5 girls and 4 boys in Homeroom A, and 3 boys and 7 girls in Homeroom B. If each homeroom selects one captain at random from among its students, what is the probability that a boy is selected in at least one of the homerooms?

67/90

11/18

11/10

12/19

7/18

11/18

Explanation:

Note that the choosing of a captain from each homeroom is independent of the other, such that the events are independent. This means that the probabilities of each event would be multiplied.

Find the probability that girls are selected for both homerooms. As this is the only possibility in which a boy is NOT selected, the answer is 1 minus this probability.

There are 9 students in Homeroom A. Of them, 5 are girls.

There are 10 students in Homeroom B. Of them, 7 are girls.

Probability of 2 girls = P(girl for A) x P(girl for B)

= (5/9) x (7/10) = 35/90 = 7/18

Probability of at least one boy = 1 - 7/18 = 11/18

### Example Question #11 : Probability

How many positive four-digit integers are multiples of 5 and less than 7,000?

800

1600

1850

1000

1200

1200

Explanation:

First, observe that multiples of 5 are all those numbers that have a 0 or a 5 as their final digit (5, 10, 15, 20, 25, etc.), so the question is asking how many 4-digit integers under 7000 end in a 0 or a 5. Second, notice that the smallest 4-digit integer is 1,000. So, to rephrase the question, we want to find how many integers that are between 1000 and 6999 end in a 0 or a 5.

Writing the 4-digit integer as WXYZ, we see that there are six possibilties for W (1, 2, 3, 4, 5, and 6), ten possibilities for X (0–9), ten possibilities for Y (0–9), and two possibilities for Z (0 and 5). So there are 6 * 10 * 10 * 2 = 1200 total integers between 1000 and 6999 that end in a 0 or a 5. Therefore, there are 1200 four-digit integers that are multiples of 5 and less than 7,000.

### Example Question #11 : How To Find The Probability Of An Outcome

A street light in Anytown, USA, completes a cycle from red to green to yellow, and back to red, in 50 seconds. During this time, the light will be red for 20 seconds, green for 25 seconds, and yellow for the remaining time. Over a period of 100 seconds, what is the probability that the light will be yellow at a randomly chosen time?

1/25

1/20

1/10

1/50

1/5

1/10

Explanation:

First, find the amount of time that the light is yellow,  which is 50 – 20 – 25 = 5 seconds out of a 50 second cycle. The period of 100 seconds is irrelevant because the probability of getting a yellow light would be the same for any complete period or number of complete periods. So the probability of getting a yellow light would be 5/50 = 1/10.

### Example Question #11 : How To Find The Probability Of An Outcome

A fair die is rolled 30 times.  How many times would you expect a 2 or a 6 to be rolled?

12

15

5

10