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

## Example Questions

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

A bag of marbles has 7 yellow marbles, 5 red marbles, 3 blue marbles, and 6 white marbles. What is the probability of choosing a yellow marble, putting it back and choosing a blue marble, and then NOT putting the blue marble back and picking a white marble?

1/9

8/31

2/3

3/17

1/70

1/70

Explanation:

There are a total of 7 + 5 + 3 + 6 = 21 marbles. The probability of picking a yellow marble is 7/21 = 1/3. Then we put it back and choose a blue marble with probability 3/21 = 1/7. We do NOT put this blue marble back, but then we grab for a white. The probability of picking a white is now 6/20 = 3/10, because now we are choosing from 20 marbles instead of 21. So putting it together, the probability of choosing a yellow marble, replacing it and then choosing a blue and a white, is 1/3 * 1/7 * 3/10 = 1/70.

### Example Question #1 : Probability

A bag of jellybeans has 20 watermelon jellybeans, 45 sour apple jellybeans, 30 orange jellybeans and 5 cotton candy jellybeans.  If you reach in and grab one jelly bean, what is the probability that it will be watermelon flavored?

1/5

1/4

4/19

1/3

1/20

1/5

Explanation:

Add up the total number of jellybeans, 20 + 45 + 30 + 5 = 100.

Divide the number of watermelon jellybeans by the total: 20/100 and reduce the fraction to 1/5.

### Example Question #136 : Probability

You went to order a cake but all of the cakes are in identical boxes. If there are five chocolate cakes, four carrot cakes, three vanilla cakes, and six red velvet cakes, what is the maximum number of boxes that you would have to open to ensure that you have one of each type of cake?

4

16

9

12

16

Explanation:

There are four different types of cake. In this type of problem we want to guarantee we have one of each, so we need to assumbe we have very bad luck. We start with the red velvet since that is the type with the most cakes. If we open those 6 we are not guaranteed to have different ones. Then say we opened all five chocolate cakes, then all four carrot cakes. We still have only three types of cakes but opened 15 boxes. When we open the next box (16) we will be guaranteed to have one of each.

### Example Question #2 : Probability

A circle is inscribed inside a square. If a point inside the square is selected at random, what is the probability that the point will also be inside the circle?

π/4

5/6

π/6

3/4

π/4

Explanation:

The probability of the point being inside the circle is the ratio of the area of the circle to the area of the square. If we suppose that the circle has radius r, then the square must have side 2r. The area of the circle is πr2 and the area of the square is 〖(2r)〗2=〖4r〗2, so the proportion of the areas is (πr2)/〖4r〗2  =π/4.

### Example Question #3 : Probability

John has a bowl with 54 marbles in it. Half of the marbles are green and half of the marbles are blue. John takes 3 green marbles and 6 blue marbles from the jar. John then takes 13 additional marbles from the remaining marbles in the jar. What is the minimum number of these 13 marbles that must be green in order for there to be more blue marbles than green marbles left?

4

9

8

10

5

9

Explanation:

The bowl has 54 marbles, half green and half blue. This gives us 27 green and 27 blue marbles:

27 G / 27 B

John then takes 3 green and 6 blue from the bowl. This leaves the bowl with:

24 G / 21 B

If there are going to be more blue than green marbles after John's 13 marbles, he has to take at least 4 more green marbles than blue marbles, because right now there are 3 less blue marbles. Therefore, we need to take at least 9 green marbles, which would mean 4 or less of the marbles would be blue (8 green and 5 blue would leave us with equal green and equal blue marbles, so it would have to be more than 8 green marbles, which gives us 9 green marbles).

We can also solve this as an inequality. You take the difference in marbles, which is 3, which means you need the difference in green and blue marbles to be greater than 3, or at least 4. You have b + g = 13 and g - b > 3, where b and g are positive integers.

b + g = 13 (Subtract g on both sides of the equation)

b = 13 - g

g - b > 3 (Substitute above equation)

g - (13 - g) > 3 (Distribute negative sign in parentheses)

g - 13 + g > 3 (Add both g variables)

2g - 13 > 3 (Add 13 to both sides of the inequality)

2g > 16 (Divide both sides of the inequality by 2)

g > 8 so g has to be 9 or greater.

### Example Question #4 : Probability

If x is chosen at random from the set (4, 6, 7, 9, 11) and y is chosen at random from the set (12, 13, 15, 17) then what is the probability that xy is odd?

9/20

11/20

9/10

6/10

3/10

9/20

Explanation:

If x is chosen at random from the set (4, 6, 7, 9, 11) and y is chosen at random from the set (12, 13, 15, 17) then what is the probability that xy is odd?

Here we have 5 possible choices for x and 4 possible choices for y, giving us 5 * 4 = 20 possible outcomes.

We know that odd times odd = odd; even times even = even; and even times odd = even. Thus we need all of the outcomes where x and y are odd. We have 3 possibilities of odd numbers for x, and 3 possibilities of odd numbers for y, so we will have 9 outcomes of our total 20 outcomes where xy is odd, giving us a probability of 9/20.

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

Mike has a bag of marbles, 4 white, 8 blue, and 6 red. He pulls out one marble from the bag and it is red. What is the probability that the second marble he pulls out of the bag is white?

4/17

4/18

1/6

3/18

4/17

Explanation:

There are 18 marbles in total. One of them is removed so now there are 17 marbles. This is our denominator. All of the original white marbles are still in the bag so there is a 4 out of 17 or 4/17 chance that the next marble taken out of the bag will be white.

### Example Question #6 : Probability

Michael tosses three fair coins. What is the probability that at least one of these coins will land on heads?

3/4

1/2

1/8

7/8

1/4

7/8

Explanation:

If Michael tosses one head, then it could be on either the first, second, or third toss. We could model it like this, where H represents heads and T represents tails.

HTT, THT, or TTH

If Michael tosses two heads, then there are three possible combinations:

HHT, HTH, or THH

If Michael tosses three heads, then there is only one possible combination:

HHH

Thus, there are seven ways that Michael can toss at least one head. We must find the probability of each of these ways and then add them together.

The probability of rolling a head is ½ and the probability of rolling a tail is ½. Because each coin toss is independent, we can multiply the probabilities together.

For example, the probability of the combination HTT is (1/2)(1/2)(1/2) = 1/8

Probability of HTT = 1/8

Probability of THT = (1/2)(1/2)(1/2) = 1/8

Probability of TTH = (1/2)(1/2)(1/2) = 1/8

Probability of HHT = 1/8

Probability of HTH = 1/8

Probability of THH = 1/8

Probability of HHH = 1/8

So, there are seven possible ways that Michael can toss at least one head. The probability of each of these seven ways is equal to 1/8. Thus, the total probability of all seven events is 7/8.

ALTERNATE SOLUTION:

Michael can toss at least one head, or he can toss zero heads. The sum of these two probabilities must equal one, because they represent all of the ways that Michael could toss the coins. He could either toss at least on head, or he could toss no heads at all.

Probability of tossing at least one head + probability of tossing no heads = 1

The probability of tossing no heads is only possible with the combination TTT. The probability of tossing three tails is equal to (1/2)(1/2)(1/2) = 1/8

Probability of tossing at least one head + 1/8 = 1

Probability of tossing at least one head = 1 – 1/8 = 7/8 .

### Example Question #7 : Probability

A jar of marbles has 125 marbles in it.  25 are blue marbles, 65 are red marble, 15 are green marbles and 20 are yellow marbles.  What is the probability that the first three marbles picked are green or blue?

0.025

0.015

0.043

0.020

0.031

0.031

Explanation:

Probability of each event = (# green marbles + # blue marbles)/ Total # of Marbles

P1 = (15 + 25) / 125 = 40 / 125

Second event assumes a blue or green was chosen for first event so there is one fewer marble on top and also one fewer marble in the total number of marbles.

P2 = (14 + 25) / 124 = 39 / 124

Third event assumes a blue or green was chosen for first and second events so there are two fewer marbles on top and also two fewer marbles in the total number of marbles.

P3 = (13 + 25) / 124 = 38 / 123

Probability for multiple events = P1 x P2 x P3

(40 / 125) * (39 / 124) * (38 / 123)

( 40 * 39 * 38) / (125 * 124 * 123 ) = 59280 / 1906500 = 0.031

### Example Question #149 : Probability

You have a full deck of 52 cards.  If there are four queens in the deck, what is the probability that out of two cards dealt to you both are queens?

0.0033

0.0052

0.0045

0.0048

0.0049

0.0045

Explanation:

Probability of each event = (# queens)/ Total # of cards

P1 = 4 / 52

Second event assumes a queen  was chosen for first event so there is one less queen and also one less card:

P2 = 3 / 51

Probability for multiple events = P1 x P2

(4 / 52) * (3 / 51)

( 4 * 3) / (52 * 51 ) = 12 / 2652 = 0.0045

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