PSAT Math : How to add odd numbers

Study concepts, example questions & explanations for PSAT Math

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Example Questions

Example Question #1 : How To Add Odd Numbers

Which of the following could represent the sum of 3 consecutive odd integers, given that d is one of the three?

Possible Answers:

3d + 3

3d – 6

3d + 4

3d – 3

3d – 9

Correct answer:

3d – 6

Explanation:

If the largest of the three consecutive odd integers is d, then the three numbers are (in descending order):

dd – 2, d – 4

This is true because consecutive odd integers always differ by two. Adding the three expressions together, we see that the sum is 3d – 6.

Example Question #1 : How To Add Odd Numbers

\dpi{100} p+r=20, where \dpi{100} p and \dpi{100} r are distinct positive integers.  Which of the following could be values of \dpi{100} p and \dpi{100} r?

Possible Answers:

0 and 20

5 and 15

4 and 5

10 and 10

–10 and 30

Correct answer:

5 and 15

Explanation:

Since \dpi{100} p and \dpi{100} r must be positive, eliminate choices with negative numbers or zero. Since they must be distinct (different), eliminate choices where \dpi{100} p=r.  This leaves 4 and 5 (which is the only choice that does not add to 20), and the correct answer, 5 and 15.

Example Question #2 : Even / Odd Numbers

The sum of three consecutive odd integers is 93. What is the largest of the integers?

Possible Answers:

Correct answer:

Explanation:

Consecutive odd integers differ by 2. If the smallest integer is x, then

x + (x + 2) + (x + 4) = 93

3x + 6 = 93

3x = 87

x = 29

The three numbers are 29, 31, and 33, the largest of which is 33.

Example Question #1 : How To Add Odd Numbers

You are given that  are all positive integers. Also, you are given that:

 is an odd number.  can be even or odd. What is known about the odd/even status of the other four numbers?

Possible Answers:

 ia odd;  and  are even;  can be either.

None of the other responses are correct.

, and  are odd;  can be either.

 and  are odd;  and  are even.

 and  are odd;  is even;  can be either.

Correct answer:

 and  are odd;  is even;  can be either.

Explanation:

The odd/even status of  is not known, so no information can be determined about that of .

 is known to be an integer, so  is an even integer. Added to odd number , an odd sum is yielded; this is .

 is known to be odd, so  is also odd. Added to odd number , an even sum is yielded; this is .

 is known to be even, so  is even. Added to odd number ; an odd sum is yielded; this is .

The numbers known to be odd are  and ; the number known to be even is ; nothing is known about .

Example Question #2 : How To Add Odd Numbers

You are given that  are all positive integers. Also, you are given that:

 is an odd number.  can be even or odd. What is known about the odd/even status of the other four numbers?

Possible Answers:

None of the other responses are correct.

, and  are odd.

 and  are even;  and  are odd.

, and  are even.

 and  are odd;  and  are even.

Correct answer:

None of the other responses are correct.

Explanation:

A power of an integer takes on the same odd/even status as that integer. Therefore, without knowing the odd/even status of , we do not know that of , and, subsequently, we cannot know that of . As a result, we cannot know the status of any of the other values of the other three variables in the subsequent statements. Therefore, none of the four choices are correct.

Example Question #1 : How To Add Odd Numbers

You are given that  are all positive integers. Also, you are given that:

You are given that  is odd, but you are not told whether  is even or odd. What can you tell about whether the values of the other four variables are even or odd?

Possible Answers:

  and  are even and  and  are odd.

 , and  are odd.

 and  are even;  is odd;  can be either.

 and  are odd;  is even;  can be either.

  and  are odd and  and  are even.

Correct answer:

  and  are odd and  and  are even.

Explanation:

, the product of an even integer and another integer, is even. Therefore,  is equal to the sum of an odd number  and an even number , and it is odd.

, the product of odd integers, is odd, so , the sum of odd integers  and , is even.

, the product of an odd integer and an even integer, is even, so , the sum of an odd integer  and even integer , is odd.

, the product of odd integers, is odd, so , the sum of odd integers  and , is even.

The correct response is that  and  are odd and that  and  are even.

Example Question #4 : How To Add Odd Numbers

, and  are positive integers. 

 is odd.

Which of the following is possible?

I) Exactly two of  are odd.

II) Exactly three of  are odd.

III) All four of  are odd.

Possible Answers:

I and III only

I and II only

I, II, and III

II and III only

None of I, II, or III

Correct answer:

I, II, and III

Explanation:

If exactly two of  are odd, then exactly one of the seven expressions being added is odd - namely, the only one that does not have an even factor (for example, if  and  are odd, then the only odd number is ). This makes  the sum of one odd number and six even number and, subsequently, odd.

 

If exactly three of  are odd, then exactly three of the seven expressions being added are odd - namely, the three that do not include the even factor (for example, if , and  are odd, then the three odd numbers are , , and ). This makes  the sum of three odd numbers and four even numbers and, subsequently, odd.

 

If all four of  are odd, then all of the seven expressions being added, being the product of only odd numbers, are odd. This makes  the sum of seven odd numbers, and, subsequently, odd.

The correct choice is that all three scenarios are possible.

Example Question #1 : Integers

Solve:  

Possible Answers:

Correct answer:

Explanation:

Add the ones digits:

Since there is no tens digit to carry over, proceed to add the tens digits:

The answer is .

Example Question #82 : Integers

At a certain high school, everyone must take either Latin or Greek. There are  more students taking Latin than there are students taking Greek. If there are  students taking Greek, how many total students are there?

Possible Answers:

Correct answer:

Explanation:

If there are  students taking Greek, then there are  or  students taking Latin. However, the question asks how many total students there are in the school, so you must add these two values together to get:

 or  total students.

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