SAT Math : Factors / Multiples

Study concepts, example questions & explanations for SAT Math

varsity tutors app store varsity tutors android store varsity tutors ibooks store

Example Questions

Example Question #11 : How To Factor A Number

If    180 = 2^{a}3^{b}5^{c}7^{d}\(\displaystyle 180 = 2^{a}3^{b}5^{c}7^{d}\), where a,b,c,d\(\displaystyle a,b,c,d\) are all positive integers, what is a+b+c+d\(\displaystyle a+b+c+d\)?

Possible Answers:

4

6

5

7

3

Correct answer:

5

Explanation:

We will essentially have to represent 180 as a product of prime factors, because 2, 3, 5, and 7 are all prime numbers. The easiest way to do this will be to find the prime factorization of 180.

180 = 18(10)= (9)(2)(10) = (3)(3)(2)(10)=(3)(3)(2)(2)(5) = 2^{2}3^{2}5^{1}\(\displaystyle 2^{2}3^{2}5^{1}\). Because 7 is not a factor of 180, we can mutiply the prime factorization of 180 by 7^{0}\(\displaystyle 7^{0}\) (which equals 1) in order to get 7 into our prime factorization.

180=\(\displaystyle 180=\) 2^23^25^17^0\(\displaystyle 2^23^25^17^0\)= 2^a3^b5^c7^d\(\displaystyle 2^a3^b5^c7^d\)

In order for 2^23^25^17^0\(\displaystyle 2^23^25^17^0\) to equal 2^a3^b5^c7^d\(\displaystyle 2^a3^b5^c7^d\), the exponents of each base must match. This means that a = 2, b = 2, c = 1, and d = 0. The sum of a, b, c, and d is 5.

The answer is 5.

Example Question #1641 : Sat Mathematics

What is the product of the distinct prime factors of 24?

Possible Answers:

\dpi{100} \small 24\(\displaystyle \dpi{100} \small 24\)

\dpi{100} \small 9\(\displaystyle \dpi{100} \small 9\)

\dpi{100} \small 5\(\displaystyle \dpi{100} \small 5\)

\dpi{100} \small 8\(\displaystyle \dpi{100} \small 8\)

\dpi{100} \small 6\(\displaystyle \dpi{100} \small 6\)

Correct answer:

\dpi{100} \small 6\(\displaystyle \dpi{100} \small 6\)

Explanation:

The prime factorization of 24 is (2)(2)(2)(3).  The distinct primes are 2 and 3, the product of which is 6.

Example Question #11 : How To Factor A Number

How many prime factors does \dpi{100} \small 2^{3}-1\(\displaystyle \dpi{100} \small 2^{3}-1\) have?

Possible Answers:

\dpi{100} \small 5\(\displaystyle \dpi{100} \small 5\)

\dpi{100} \small 0\(\displaystyle \dpi{100} \small 0\)

\dpi{100} \small 2\(\displaystyle \dpi{100} \small 2\)

\dpi{100} \small 3\(\displaystyle \dpi{100} \small 3\)

\dpi{100} \small 1\(\displaystyle \dpi{100} \small 1\)

Correct answer:

\dpi{100} \small 1\(\displaystyle \dpi{100} \small 1\)

Explanation:

\dpi{100} \small 2^{3}-1=8-1=7\(\displaystyle \dpi{100} \small 2^{3}-1=8-1=7\)

Since 7 is prime, its only prime factor is itself.

Example Question #12 : How To Factor A Number

What is the smallest positive multiple of 12?

Possible Answers:

\dpi{100} \small 12\(\displaystyle \dpi{100} \small 12\)

\dpi{100} \small 0\(\displaystyle \dpi{100} \small 0\)

\dpi{100} \small 2\(\displaystyle \dpi{100} \small 2\)

\dpi{100} \small 24\(\displaystyle \dpi{100} \small 24\)

\dpi{100} \small 6\(\displaystyle \dpi{100} \small 6\)

Correct answer:

\dpi{100} \small 12\(\displaystyle \dpi{100} \small 12\)

Explanation:

Multiples of 12 are found by multiplying 12 by a whole number.  Some examples include:

\dpi{100} \small 12(-2)=-24\(\displaystyle \dpi{100} \small 12(-2)=-24\)

\dpi{100} \small 12(0)=0\(\displaystyle \dpi{100} \small 12(0)=0\)

\dpi{100} \small 12(1)=12\(\displaystyle \dpi{100} \small 12(1)=12\)

Clearly, the smallest positive value obtainable is 12.  Do not confuse the term multiple with the term factor!

Example Question #61 : Factors / Multiples

How many prime factors of 210 are greater than 2?

Possible Answers:

four

two

five

three

one

Correct answer:

three

Explanation:

Begin by identifying the prime factors of 210. This can be done easily using a factoring tree (see image).

Vt_p2

 

 The prime factors of 210 are 2, 3, 5 and 7. Of these factors, three of them are greater than 2. 

Example Question #12 : How To Factor A Number

How many integers between 50 and 100 are divisible by 9?

Possible Answers:

6

9

7

5

8

Correct answer:

6

Explanation:

The smallest multiple of 9 within the given range is \inline \dpi{200} \tiny 54 = 9 \times 6\(\displaystyle \inline \dpi{200} \tiny 54 = 9 \times 6\).

The largest multiple of 9 within the given range is \dpi{100} {99=9 \times 11}\(\displaystyle \dpi{100} {99=9 \times 11}\).

Counting the numbers from 6 to 11, inclusive, yields 6.

Example Question #11 : Other Factors / Multiples

 \(\displaystyle N\) is the set of all positive multiples of \(\displaystyle 5\), and \(\displaystyle W\) is the set of all squares of integers. Which of the following numbers belongs to both sets?

Possible Answers:

\(\displaystyle 1025\)

\(\displaystyle 225\)

\(\displaystyle 20\)

\(\displaystyle 150\)

\(\displaystyle 75\)

Correct answer:

\(\displaystyle 225\)

Explanation:

\(\displaystyle 225\) is the only choice that is both a multiple of \(\displaystyle 5\) and a perfect square. 

\(\displaystyle 5\cdot 45=225\)

\(\displaystyle \sqrt{225}=15\)

Learning Tools by Varsity Tutors