GRE Subject Test: Math : Probability & Statistics

Study concepts, example questions & explanations for GRE Subject Test: Math

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Example Questions

Example Question #1 : Probability & Statistics

A student has 14 piece of gum, 3 are spearmint, 5 are peppermint, and the rest are cinnamon. If one piece of gum is chosen at random, which of the following is NOT true.

Possible Answers:

The probability of picking a cinnamon or a peppermint is \(\displaystyle 11/14\).

The probability of picking a spearmint is \(\displaystyle 3/14\).

The probability of not picking a cinnamon is \(\displaystyle 4/7\).

The probability of picking a cinnamon is \(\displaystyle 3/7\).

The probability of picking a spearmint or a cinnamon is \(\displaystyle 1/2\).

Correct answer:

The probability of picking a spearmint or a cinnamon is \(\displaystyle 1/2\).

Explanation:

The probability of picking a spearmint or a cinnamon is the addition of probability of picking a spearmint and the probability of picking a cinnamon

\(\displaystyle \frac{3}{14} + \frac{6}{14} = \frac{9}{14}\) not \(\displaystyle \frac{1}{2}\)

Example Question #1 : Other Topics

If I toss a coin 3 times, how many times will I roll at least one head?

Possible Answers:

\(\displaystyle \frac{7}{8}\)

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

\(\displaystyle \frac{1}{8}\)

None of the Above

Correct answer:

\(\displaystyle \frac{7}{8}\)

Explanation:

Step 1: We need to find out how many outcomes there will be.
If we roll a coin three times, there are \(\displaystyle 2^3\) outcomes.
If we roll a coin \(\displaystyle n\) times, there will be \(\displaystyle 2^n\) outcomes.

Step 2: Find all the outcomes.

The outcomes here are: HHH, HHT, HTH, HTT, THH, THT, TTH, and TTT.

Step 3: In the list of outcomes, count how many times the letter H appears at least once.

The letter "H" appears in HHH, HHT, HTH, HTT, THH, THT, and TTH.

The letter "H" appears in \(\displaystyle 7\) of the \(\displaystyle 8\) outcomes.

The probability of getting at least one H in the outcomes in Step 2 is \(\displaystyle \frac{7}{8}\).


Example Question #1 : Probability

How many different combinations can i have when flipping a coin three times?

Possible Answers:

\(\displaystyle \large 16\)

\(\displaystyle 8\)

\(\displaystyle 2\)

\(\displaystyle \large 4\)

Correct answer:

\(\displaystyle 8\)

Explanation:

Step 1: Let's answer a smaller problem. How many ways can I toss one coin?

There are two ways, either I get Heads or Tails.

Step 2: How about two coins?

There are four ways... They are, HH, HT, TH, and TT

Step 3: How many different combinations for three coins?
Let's List them:
HHH, HHT, HTH, THH, TTT, THT, TTH, HTT

There are \(\displaystyle \large 8\) different combinations.

Example Question #1 : Probability & Statistics

Find the mean of the following set of numbers:

\(\displaystyle S=(13,14,177,65,87,23,45,78,23)\)

 

Possible Answers:

\(\displaystyle 48.\bar{3}\)

\(\displaystyle 87\)

\(\displaystyle 33.\bar{3}\)

\(\displaystyle 58.\bar{3}\)

\(\displaystyle 28.\bar{3}\)

Correct answer:

\(\displaystyle 58.\bar{3}\)

Explanation:

The mean can be found in the same way as the average of a group of numbers. To find the average, use the following formula:

\(\displaystyle Mean=\frac{\textup{sum of data points}}{\textup{total number of data points}}\)

So, if our set consists of 

\(\displaystyle S=(13,14,177,65,87,23,45,78,23)\)

We will get our mean via:

\(\displaystyle Mean=\frac{13+14+177+65+87+23+45+78+23}{9}=58.\bar{3}\)

So our answer is

\(\displaystyle 58.\bar{3}\)

Example Question #1 : Other Topics

The mean of four numbers is \(\displaystyle 35\).

A: The sum of the four numbers.

B: \(\displaystyle 140\)

Possible Answers:

Quantity A is greater.

Quantity B is greater.

Can't be determined from the given information.

Both are equal.

Correct answer:

Both are equal.

Explanation:

To find the sum of the four numbers, just multiply four and the average. By multiplying the average and number of terms, we get the sum of the four numbers regardless of what those values could be.

\(\displaystyle 35*4=140\) Since Quantity A matches Quantity B, answer should be both are equal. 

Example Question #2 : Statistics

Mean of \(\displaystyle x, y, z\) is \(\displaystyle 60\)\(\displaystyle x, y, z\) are all positive integers. \(\displaystyle z\) is between \(\displaystyle 50\) and \(\displaystyle 70\) inclusive. 

A: Mean of \(\displaystyle x,y\).

B: Mean of \(\displaystyle y,z\)

Possible Answers:

Both are equal. 

Can't be determined from the information above.

Quantity A is greater.

Quantity B is greater.

Correct answer:

Can't be determined from the information above.

Explanation:

Let's look at a case where \(\displaystyle z=70\).

Let's have \(\displaystyle y\) be \(\displaystyle 109\) and \(\displaystyle x\) be \(\displaystyle 1\). The sum of the three numbers have to be \(\displaystyle 3*60\) or \(\displaystyle 180\)

The average of \(\displaystyle x,y\) is \(\displaystyle \frac{110}{2}\) or \(\displaystyle 55\). The avergae of \(\displaystyle y,z\) is \(\displaystyle \frac{179}{2}\) or \(\displaystyle 89.5\).

This makes Quantity B bigger, HOWEVER, what if we switched the \(\displaystyle y\) and \(\displaystyle x\) values. 

The average of \(\displaystyle x,y\) is still \(\displaystyle \frac{110}{2}\) or \(\displaystyle 55\). The avergae of \(\displaystyle y,z\) is \(\displaystyle \frac{71}{2}\) or \(\displaystyle 35.5\).

This makes Quantity A bigger. Because we have two different scenarios, this makes the answer can't be determined based on the information above.

Example Question #1 : Probability & Statistics

If \(\displaystyle x>y\) and are positive integers from \(\displaystyle 6-10\) inclusive, then:

A: The mean of \(\displaystyle 2, 3, 7, 4, x\)

B: The mean of \(\displaystyle 1, 6, 5, 8, y\)

Possible Answers:

Quantity A is greater

Can't be determined from the information above

Both are equal

Quantity B is greater

Correct answer:

Can't be determined from the information above

Explanation:

Let's add each expression from each respective quantity

Quantity A: \(\displaystyle 2+3+7+4+x=16+x\)

Quantity B: \(\displaystyle 1+6+5+8+y=20+y\)

Since \(\displaystyle x>y\) we will let \(\displaystyle x=10\) and \(\displaystyle y=6\). The sum of Quantity A is \(\displaystyle 26\) and the sum of Quantity B is also \(\displaystyle 26\). HOWEVER, if \(\displaystyle y\) was \(\displaystyle 7\), that means the sum mof Quantity B is \(\displaystyle 27\). With the same number of terms in both quantities, the larger sum means greater mean. First scenario, we would have same mean but the next scenario we have Quantity B with a greater mean. The answer is can't be determined from the information above. 

Example Question #501 : Gre Subject Test: Math

John picks five numbers out of a set of seven and decides to find the average. The set has \(\displaystyle 0, 5, 8, 2, 9, 4, 7\)

A: John averages the five numbers he picked from the set.

B: \(\displaystyle 7\)

Possible Answers:

Quantity A is greater

Both are equal

Quantity B is greater

Can't be determined from the information above

Correct answer:

Quantity B is greater

Explanation:

To figure out which Quantity is greater, let's find the highest possible mean in Quantity A. We should pick the \(\displaystyle 5\) biggest numbers which are \(\displaystyle 4, 5, 7, 8, 9\). The mean is \(\displaystyle \frac{4+5+7+8+9}{5}=\frac{33}{5}=6.6\). This is the highest possible mean and since Quantity B is \(\displaystyle 7\) this makes Quantity B is greater the correct answer.

Example Question #2 : Probability & Statistics

Find the mean.

\(\displaystyle 5, 10, 18, 96\)

Possible Answers:

\(\displaystyle 32.25\)

\(\displaystyle 14\)

\(\displaystyle 129\)

\(\displaystyle 33.25\)

Correct answer:

\(\displaystyle 32.25\)

Explanation:

To find the mean, add the terms up and divide by the number of terms.

\(\displaystyle \frac{5+10+18+96}{4}=\frac{129}{4}=33.25\)

Example Question #2 : Other Topics

Find \(\displaystyle x\)  if the mean of \(\displaystyle 7, 15, 25, 36, x\) is \(\displaystyle 20\).

Possible Answers:

\(\displaystyle 17\)

\(\displaystyle 83\)

\(\displaystyle 20\)

\(\displaystyle -7\)

Correct answer:

\(\displaystyle 17\)

Explanation:

To find the mean, add the terms up and divide by the number of terms.

\(\displaystyle \frac{7+15+25+36+x}{5}=20\) Then add the numerator.

\(\displaystyle \frac{83+x}{5}=20\) Cross-multiply.

\(\displaystyle 83+x=100\) Subtract \(\displaystyle 83\) on both sides.

\(\displaystyle x=17\)

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