ISEE Lower Level Math : How to find the probability of an outcome

Study concepts, example questions & explanations for ISEE Lower Level Math

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Example Questions

Example Question #21 : How To Find The Probability Of An Outcome

Jen has a bag of 16 marbles. In the bag, there are 8 green marbles, 4 white, and 4 red marbles. If Jen reaches in and randomly chooses a marble, what is the probability (expressed as a percentage) that she chooses a white marble?

Possible Answers:

\dpi{100} 40\%\(\displaystyle \dpi{100} 40\%\)

\dpi{100} 75\%\(\displaystyle \dpi{100} 75\%\)

\dpi{100} 25\%\(\displaystyle \dpi{100} 25\%\)

\dpi{100} 50\%\(\displaystyle \dpi{100} 50\%\)

\(\displaystyle 20\%\)

Correct answer:

\dpi{100} 25\%\(\displaystyle \dpi{100} 25\%\)

Explanation:

The chance of choosing a white marble is \(\displaystyle 4\) out of \(\displaystyle 16\), or \dpi{100} \frac{1}{4}\(\displaystyle \dpi{100} \frac{1}{4}\)

 \dpi{100} \frac{1}{4}\(\displaystyle \dpi{100} \frac{1}{4}\) expressed as a percentage is \(\displaystyle 25\%\)

Example Question #21 : How To Find The Probability Of An Outcome

Use the diagram to answer the question.

Probability

 

If one of the shapes is chosen at random, what is the probability that it is a black circle?

Possible Answers:

1 out of 6

1 out of 12

1 out of 5

1 out of 4

1 out of 8

Correct answer:

1 out of 6

Explanation:

To find the probability, we use a fraction:

\(\displaystyle \frac{part}{total}\)

There are 12 total shapes, and 2 black circles:

\(\displaystyle \frac{2}{12}\)

Reduce:

\(\displaystyle \frac{1}{6}\)

Thus, the probability is 1 out of 6.

Example Question #22 : Probability

A pencil bag holds 4 pens, 3 pencils, and 3 highlighters. What is the probability of picking out a pen from the bag without looking?

Possible Answers:

\(\displaystyle \frac{3}{10}\)

\(\displaystyle \frac{7}{10}\)

\(\displaystyle \frac{2}{5}\)

\(\displaystyle \frac{1}{5}\)

\(\displaystyle \frac{1}{}4\)

Correct answer:

\(\displaystyle \frac{2}{5}\)

Explanation:

Probability involves part over whole. First, add up how many items are in the bag. There are 10 items. Then, count how many pens there are (4). Put 4 over 10 to get \(\displaystyle \frac{4}{10}\). Then, you must simplify it to find the right answer. 2 goes into both the numerator and denominator, so it can be reduced to \(\displaystyle \frac{2}{}5\).

Example Question #561 : Isee Lower Level (Grades 5 6) Mathematics Achievement

Ana put 5 black marbles, 3 blue marbles, and 2 green marbles into a bag. She then picked one marble out. What is the probability that the marble was black?

Possible Answers:

\(\displaystyle 20\%\)

\(\displaystyle 30\%\)

\(\displaystyle 50\%\)

\(\displaystyle 75\%\)

\(\displaystyle 60\%\)

Correct answer:

\(\displaystyle 50\%\)

Explanation:

Given that there are 5 black marbles, 3 blue marbles, and 2 green marbles, there are a total of 10 marbles. 

\(\displaystyle 5+3+2=10\)

If Ana picks a marble, there are 5 marbles that are black. Thus, there would a 5 out of 10 chance that she would get a black marble.

\(\displaystyle \frac{5\ \text{black}}{10\ \text{total}}=\frac{5}{10}\)

We can convert to a percent by multiplying to get 100 in the denominator.

\(\displaystyle \frac{5}{10}=\frac{5\times10}{10\times10}=\frac{50}{100}\)

Fifty over one hundred is equal to \(\displaystyle 50\%\). There is a \(\displaystyle 50\%\) chance that she will get a black marble because half of the marbles in the bag are black.

Example Question #563 : Isee Lower Level (Grades 5 6) Mathematics Achievement

There are 11 discs in a box, each one labeled with a number 1 through 11. Beth picks a disc out of the box. The first disc is the number 9. Beth is going to pick another disc. What is the likelihood that she will pick an odd disc next?

Possible Answers:

\(\displaystyle 10\%\)

\(\displaystyle 100\%\)

\(\displaystyle 50\%\)

\(\displaystyle 25\%\)

\(\displaystyle 0\%\)

Correct answer:

\(\displaystyle 50\%\)

Explanation:

Initially the discs in the box were: \(\displaystyle 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11.\)

6 of these numbers are odd and 5 are even.

When Beth picked disc number 9, there remained 5 odd discs and 5 even discs. 

Odd discs remaining: \(\displaystyle 1,3,5,7,11\)

There are ten total discs remaining. To find the probability of an odd disc:

\(\displaystyle \frac{\text{odd}}{\text{total}}=\frac{5}{10}\)

This fraction is equal to one half, or \(\displaystyle 50\%\).

Example Question #23 : Probability

Andrew has three quarters. If he flips the three quarters, what is the probability that the last quarter will be heads?

Possible Answers:

33 percent

66 percent

50 percent

70 percent

Correct answer:

50 percent

Explanation:

When flipping a coin, the possibility that it will be heads or tails will always be 50/50. Even though Andrew flips the coin twice before the third flip, the prior flips do not affect the outcome of the third flip. 

Example Question #24 : Probability

Molly baked a pie with 6 slices. 3 slices have a sugar topping, 1 slice has a fruit topping, and 2 slices have no topping. What percentage of the pie has a sugar topping?

Possible Answers:

\(\displaystyle 33\%\)

\(\displaystyle 17\%\)

\(\displaystyle 50\%\)

\(\displaystyle 30\%\)

\(\displaystyle 60\%\)

Correct answer:

\(\displaystyle 50\%\)

Explanation:

The Molly baked a pie with 6 slices. 3 slices have a sugar topping, 1 slice has a fruit topping, and 2 slices have no topping. Therefore, the proportion of sugar slices to the rest of the pie is:

\(\displaystyle \frac{\text{sugar}}{\text{total}}=\frac{3}{6}=\frac{1}{2}\)

Given that 50% is the percent equivalent of \(\displaystyle \frac{1}{2}\), that is the correct answer.

Example Question #22 : Probability

In a turtle race, five turtles sprinted towards the finish line. Mary chose a turtle at random before the race. What is the chance that her turtle will finish in first or second place?

Possible Answers:

\(\displaystyle 20\%\)

\(\displaystyle 40\%\)

\(\displaystyle 60\%\)

\(\displaystyle 25\%\)

\(\displaystyle 45\%\)

Correct answer:

\(\displaystyle 40\%\)

Explanation:

If there are 5 turtles racing, there is a 2 out of 5 chance that a certain turtle will be first or second place.

\(\displaystyle \frac{2\ \text{options}}{5\ \text{total}}\)

Convert this value to a decimal.

\(\displaystyle \frac{2}{5}=\frac{4}{10}=0.40\)

Convert the decimal to a percentage.

\(\displaystyle 0.40=40\%\)

Example Question #22 : Data Analysis And Probability

Gregory is answering a multiple choice question that has 5 answer choices. He correctly eliminates 2 of the options, and then guesses. What chance does he have of answering the question correctly?

Possible Answers:

\(\displaystyle \frac{1}{3}\)

\(\displaystyle \frac{1}{2}\)

\(\displaystyle \frac{1}{4}\)

\(\displaystyle \frac{1}{6}\)

\(\displaystyle \frac{1}{5}\)

Correct answer:

\(\displaystyle \frac{1}{3}\)

Explanation:

If there are 5 answer choices, and Gregory eliminates 2 of the answer choices, then there are 3 answer options left.

\(\displaystyle 5-2=3\)

Of the 3 total options left, 1 of them is correct.

\(\displaystyle \frac{1\ \text{correct}}{3\ \text{total}}=\frac{1}{3}\)

If he guesses, there is a \(\displaystyle \frac{1}{3}\) chance that he will answer the question correctly because he is guessing among 3 options. Therefore, the correct answer is \(\displaystyle \frac{1}{3}\).

Example Question #24 : Outcomes

Jenny has 4 puppies that she is trying to get adopted. 3 of them have spots. One of the puppies is adopted, at random, from the group. What are the odds that the adopted puppy had spots?

Possible Answers:

\(\displaystyle \frac{3}{4}\)

\(\displaystyle \frac{1}{2}\)

\(\displaystyle \frac{1}{4}\)

\(\displaystyle \frac{2}{3}\)

\(\displaystyle \frac{1}{5}\)

Correct answer:

\(\displaystyle \frac{3}{4}\)

Explanation:

To find the chance, we need to set up a fraction that looks like this:

\(\displaystyle \frac{\text{puppies with spots}}{\text{total puppies}}\)

Using the information from the question, we know that this fraction is equal to:

\(\displaystyle \frac{\text{puppies with spots}}{\text{total puppies}}=\frac{3}{4}\)

There is a \(\displaystyle \frac{3}{4}\) chance that the puppy picked will have spots.

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