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

## Example Questions

### Example Question #71 : How To Find The Probability Of An Outcome

How many different ways can 15 runners receive ribbons in a race if 1st, 2nd, 3rd, and 4th place ribbons are given?      Explanation:

Order matters, so a permutation of 15 things chosen 4 at a time, or , is the correct answer.

### Example Question #72 : How To Find The Probability Of An Outcome

A bag of marbles consists of 13 green, 15 blue, 12 black and an unknown number of red marbles. If the chance of choosing a red marble is 1 in 3, how many marbles are there total?      Explanation:

Adding the green, blue, and black marbles together gives 40. Then if there is a chance that a red marble is drawn, there must be an additional 20 marbles.

### Example Question #73 : How To Find The Probability Of An Outcome

Tiffany picks a peice of fruit from a bowl at random.  The bowl contains 2 red delicious apples, 3 granny smith apples, 1 orange and 3 grapefruits.  What is the probability that she chooses an apple?      Explanation:

The chance she picks an apple is the total number of apples over the total number of fruits in the bowl.  Since there are 2 kinds of apples, we add 2 red delicious and 3 granny smiths to get 5 apples total.  In addition there are 4 more peices of fruit, so the total number of fruits is 9.

### Example Question #74 : How To Find The Probability Of An Outcome

In a bag, there are 6 black marbles and 8 yellow marbles. What is the possibility of choosing a yellow marble at random?     Explanation:

There is a total of 14 marbles, of which, 8 are yellow; therefore the probability of choosing a yellow marble is or ### Example Question #75 : How To Find The Probability Of An Outcome

In a bag, there are 6 black and 8 yellow marbles. A yellow marble is chosen at random and is not replaced. What is the probability of choosing a second yellow marble?     Explanation:

Because a yellow marble was selected and not replaced, there is a total of 13 marbles in the bag and of which, 7 are yellow; therefore, the probability of selecting a yellow marble is ### Example Question #76 : How To Find The Probability Of An Outcome

A couple has four children.  What is the probability of having exactly three girls?      Explanation:

The total number of possibiliities is 16.  The outcome we are looking for is three girls and one boy which happens only four times.  Thus the probability becomes .

### Example Question #71 : How To Find The Probability Of An Outcome

When a penny is flipped three times in a row, what is the probability of getting all heads or all tails?      Explanation:

The sample space for three coin tosses in a row is 8.  Getting all heads can be done only one way.  Getting all tails can be done only one way.  So the probability of getting all heads or all tails is or .

### Example Question #77 : How To Find The Probability Of An Outcome

A couple has three children.  What is the probability of having exactly two girls or two boys?      Explanation:

The sample space for three children is 8.  We can count up what we want or subtract out what we don't want.  What we don't want is all boys or all girls.

So the probability becomes ### Example Question #1531 : Psat Mathematics

How many different ways can five books be lined up on a shelf?      Explanation:

Order matters, so we use permutations. is ### Example Question #78 : How To Find The Probability Of An Outcome

Jimmy is having trouble choosing what to wear to the baseball game.  He has six shirts, three pants, two pairs of socks and three pairs of shoes.  How many different outfits does Jimmy have to choose from?      Explanation:

Choosing one of each clothing item is an independent event and should be multiplied together.  So the answer becomes or different combinations.

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