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Prime numbersUnderstanding multiplication

ExponentsHere you will learn about the least common multiple, including a review on prime factorization and how to use it to find the least common multiple

Students will first learn about the least common multiple as part of the number system in 6th grade.

The **least common multiple** (LCM) is the smallest number that two or more integers share as a multiple. The name itself tells you what the skill is:

Take, for example, 4 and 6.

By listing the multiples of 4 and 6, you can see that the first common multiple that occurs in each list is 12. So, the **least common multiple** of 4 and 6 is 12.

You may also notice that 24 occurs in both lists. This is a common multiple, but not the least common multiple.

Calculating the** least common multiple **becomes more complicated for larger numbers. Listing all the multiples of each number can be time-consuming and it is easy to miscalculate a multiple.

To make it simpler, you can use the **prime factors** of both numbers. You can use **prime factors** and a **Venn diagram** to calculate the **Least Common Multiple.**

You need to write out the prime factorization of each number fully and then put the numbers into the Venn diagram by looking for pairs.

Here, you have the prime factors of 12 and 30.

- The yellow section represents the prime factors of 12 that are not prime factors of 30 (which is just the prime factor of 2).
- The blue section represents the prime factors of 30 that are not prime factors of 12 (which is just the prime factor of 5).
- The green intersection represents the prime factors of 12 and 30 \; (2 and 3).

The **least common multiple** of 12 and 30 is equal to the **product of all the factors viewed in the Venn diagram**.

The least common multiple is 2 \times 2 \times 3 \times 5= 60.

How does this relate to 6th grade math?

**Grade 6 – The Number System (6.NS.B.4)**Find the greatest common factor of two whole numbers less than or equal to 100 and the least common multiple of two whole numbers less than or equal to 12. Use the distributive property to express a sum of two whole numbers

In order to calculate the least common multiple for two or more numbers:

**State the product of prime factors of each number.****Write all the prime factors into the Venn diagram for each number.****Multiply each prime factor in the Venn diagram to find the LCM.**

Use this quiz to check your grade 4 to 6 students’ understanding of factors and multiples. 10+ questions with answers covering a range of 4th, 5th and 6th grade topics to identify areas of strength and support!

DOWNLOAD FREEUse this quiz to check your grade 4 to 6 students’ understanding of factors and multiples. 10+ questions with answers covering a range of 4th, 5th and 6th grade topics to identify areas of strength and support!

DOWNLOAD FREECalculate the least common multiple of 18 and 24.

**State the product of prime factors of each number.**

\begin{aligned} & 18=2 \times 3 \times 3 \\\\ & 24=2 \times 2 \times 2 \times 3 \end{aligned}

2**Write all the prime factors into the Venn diagram for each number.**

3**Multiply each prime factor in the Venn diagram to find the LCM.**

LCM =3 \times 2 \times 3 \times 2 \times 2=72

Calculate the least common multiple of 14 and 56.

**State the product of prime factors of each number.**

\begin{aligned} & 14=2 \times 7 \\\\ & 56=2 \times 2 \times 2 \times 7 \end{aligned}

**Write all the prime factors into the Venn diagram for each number.**

**Multiply each prime factor in the Venn diagram to find the LCM.**

LCM =2 \times 7 \times 2 \times 2=56

Given that 90=2 \times 3^2 \times 5 in exponent form, work out the least common multiple of 54 and 90.

**State the product of prime factors of each number.**

To state the product of prime factors of 54, you use a factor tree:

\begin{aligned} & 54=2 \times 3 \times 3 \times 3 \\\\
& 90=2 \times 3 \times 3 \times 5 \end{aligned}

**Write all the prime factors into the Venn diagram for each number.**

**Multiply each prime factor in the Venn diagram to find the LCM.**

LCM =3 \times 2 \times 3 \times 3 \times 5=270

Calculate the least common multiple of 12, 20, and 32.

**State the product of prime factors of each number.**

\begin{aligned} & 12=2 \times 2 \times 3 \\\\ & 20=2 \times 2 \times 5 \\\\ & 32=2 \times 2 \times 2 \times 2 \times 2 \end{aligned}

**Write all the prime factors into the Venn diagram for each number.**

**Multiply each prime factor in the Venn diagram to find the LCM.**

LCM =2 \times 2 \times 2 \times 2 \times 2 \times 3 \times 5=480

Calculate the least common multiple of 35 and 72.

**State the product of prime factors of each number.**

\begin{aligned} & 35=5 \times 7 \\\\ & 72=2 \times 2 \times 2 \times 3 \times 3 \end{aligned}

**Write all the prime factors into the Venn diagram for each number.**

**Multiply each prime factor in the Venn diagram to find the LCM.**

LCM =5 \times 7 \times 2 \times 2 \times 2 \times 3 \times 3=35 \times 72=2,520

There are two lights blinking on a machine. The green light blinks every 36 seconds and the blue light blinks every 42 seconds. When they both blink at the same time, how much time is it before they blink at the same time again?

**State the product of prime factors of each number.**

\begin{aligned} & 36=2 \times 2 \times 3 \times 3 \\\\ & 42=2 \times 3 \times 7 \end{aligned}

**Write all the prime factors into the Venn diagram for each number.**

**Multiply each prime factor in the Venn diagram to find the LCM.**

LCM =2 \times 3 \times 2 \times 3 \times 7=252 seconds

To answer the question:

252 seconds = 4 minutes and 12 seconds, so they blink at the same time every 4 minutes and 12 seconds.

- At the beginning, use smaller numbers so students can focus on learning and understanding the procedure. Then move to larger numbers to challenge students’ ability to find all the prime factors of a number.

- Look for opportunities for students to use LCM in real-world situations instead of using just practice problem worksheets.

- Some students may struggle to find all the prime factors, especially for larger numbers. Provide a calculator to assist students so that they can keep their focus on the skill and not be held back by the calculations.

**Calculating the greatest common factor (GCF) instead of LCM**

It is common to mix up the least common multiple with the greatest common factor. To avoid this, remember the definition of multiples and factors.

Multiples: the product of two or more factors being multiplied

Factors: the number being multiplied

For example,

What is the GCF of 3 and 9? What is the LCM of 3 and 9?

Factors of 3\text{: } 1, 3 Multiples of 3\text{: } 3, 6, 9, 12, 15…

Factors of 9\text{: } 1, 3, 9 Multiples of 9\text{: } 9, 18, 27, 36…

The GCF is 3 The LCM is 9

**Multiplying the two numbers and assuming that is the LCM**

Multiplying the two numbers in the question will find a common multiple, but it may not be the least common multiple.

For example,

What is the LCM of 3 and 9?

3 \times 9 = 27 ← common multiple, but not the LCM

**Not writing duplicate factors in the Venn diagram**

It is easy to forget the repeated prime factors in a Venn diagram, but the procedure only works if they are all included.

For example,

What is the LCM of 12 and 20?

1. Calculate the least common multiple of 42 and 70.

2,940

210

14

420

\begin{aligned}
&42=2 \times 3 \times 7\\\\
&70=2 \times 5 \times 7
\end{aligned}

LCM =2\times 3 \times 5 \times 7=210

2. Calculate the least common multiple of 38 and 76.

38

2,888

76

19

\begin{aligned}
&38= 2 \times 19 \\\\
&76= 2\times 2 \times 19
\end{aligned}

LCM =2 \times 2 \times 19 =76

3. Given that 68=2^{2} \times 17, calculate the least common multiple of 24 and 68.

408

1632

4

816

24=2 \times 2 \times 2 \times 3

LCM =2 \times 2 \times 2 \times 3 \times 17=408

4. Calculate the least common multiple of 10, 25, and 45.

11,250

5

450

2,250

\begin{aligned}
&10=2 \times 5\\\\
&25=5 \times 5\\\\
&45=3 \times 3 \times 5
\end{aligned}

LCM =2 \times 3 \times 3 \times 5 \times 5 = 450

5. Calculate the least common multiple of 16 and 45.

144

1,440

1

720

\begin{aligned}
&16=2 \times 2 \times 2 \times 2\\\\
&45=3 \times 3 \times 5
\end{aligned}

LCM =2 \times 2 \times 2 \times 2 \times 3 \times 3 \times 5 =720

6. Two buckets of water are leaking. Bucket A drips water every 16 seconds, while bucket B drips water every 18 seconds. If they both drip water at the same time, what is the length of time between them both dripping water again at the same time?

144 mins

2 secs

1 min 12 secs

2 mins 24 secs

\begin{aligned}
&16=2 \times 2 \times 2 \times 2\\\\
&18=2 \times 3 \times 3
\end{aligned}

LCM =2 \times 2 \times 2 \times 2 \times 3 \times 3=144

144 seconds = 2 mins 24 secs

The Venn diagram breaks each number down into its prime factors. Writing numbers once in the intersection eliminates all the duplicate factors, so what is left are the prime factors of the smallest multiple the numbers have in common. So multiplying them together equals the LCM.

Lowest Common Denominator (LCD), Least Common Denominator (LCD), and Lowest Common Multiple (LCM) are also other terms that mean the same as Least Common Multiple.

Yes, besides the prime factorization method, you can make a list of multiples for each number until you reach the first common multiple.

No, not necessarily. For example, the number 36=2 \times 2 \times 3 \times 3 has 4 prime factors while 71 = 3 \times 17 has only 2, so in this case the smallest number has more factors.

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