1. 5-1 Least Common Multiple Warm Up Problem of the Day
Course 1
Warm Up
Problem of the Day
Lesson Presentation

2. Warm Up
Write the first five multiples of each number.
1. 5
2. 6
3. 10
4. 12
5, 10, 15, 20, 25
6, 12, 18, 24, 30
10, 20, 30, 40, 50
12, 24, 36, 48, 60

3. Problem of the Day
Greg, Sam and Mary all work at the same high school. One of them is a principal, one of them is a teacher, and one of them is a janitor. Sam is older than Mary. Mary does not live in the same town as the principal. The teacher, the oldest of the three, often plays golf with Greg. What is each person’s job?
Greg, principal; Sam, teacher; Mary, janitor

4. Learn to find the least common multiple (LCM) of a group of numbers.

5. Vocabulary
least common multiple (LCM)

6. A multiple of a number is the product of the number and any nonzero whole number.
Remember!

7. Additional Example 1: Consumer Application
English muffins come in packs of 8, and eggs come in cartons of 12. If there are 24 students, what is the least number of packs and cartons needed so that each student has a muffin sandwich with one egg and there are no muffins left over?
Draw muffins in groups of 8. Draw eggs in groups of 12. Stop when you have drawn the same number of each.
There are 24 English muffins and 24 eggs.
So 3 packs of English muffins and 2 cartons of eggs are needed.

8. Check It Out: Example 1
Dog cookies come in packages of 6, and bones in bags of 9. If there are 18 dogs, what is the least number of packages and bags needed so that each dog has a treat box with one bone and one cookie and there are no bones or cookies left over?
Draw cookies in groups of 6. Draw bones in groups of 9. Stop when you have drawn the same number of each.
There are 18 dog cookies and 18 bones.
So 3 packages of dog cookies and 2 bags of bones are needed.

9. The smallest number that is a multiple of two or more numbers is the least common multiple (LCM). In Additional Example 1, the LCM of 8 and 12 is 24.

10. Additional Example 2A: Using Multiples to Find the LCM
Find the least common multiple (LCM).
Method 1: Use a number line.
3 and 4
Use a number line to skip count by 3 and 4.
The least common multiple (LCM) of 3 and 4 is 12.

11. Additional Example 2B: Using Multiples to Find the LCM
Find the least common multiple (LCM).
Method 2: Use a list.
4, 5, and 8
4: 4, 8, 12 , 16, 20, 24, 28, 32, , 40, 44, . . .
List multiples of 4, 5, and 8.
5: 5, 10, 15, 20, 25, 30, 35, 40, 45, . . .
8: 8, 16, 24, 32, 40, 48, . . .
Find the smallest number that is in all the lists.
LCM: 40

12. The prime factorization of a number is the number written as a product of its prime factors.
Remember!

13. Additional Example 2C: Using Multiples to Find the LCM
Find the least common multiple (LCM).
Method 3: Use prime factorization.
6 and 20
6 = 2 • 3
Write the prime factorization of each number.
20 = • 2 • 5
Line up the common factors.
2 • 3 • 2 • 5
To find the LCM, multiply one number from each column.
2 • 3 • 2 • 5 = 60
LCM: 60

14. Additional Example 2D: Using Multiples to Find the LCM
Find the least common multiple (LCM).
15, 6, and 4
15 = 3 • 5
Write the prime factorization of each number in exponential form.
6 = 3 • 2
4 =
To find the LCM, multiply each prime factor once with the greatest exponent used in any of the prime factorizations.
3 • 5 • 22
3 • 5 • 22 = 60
LCM: 60

15. Find the least common multiple (LCM). Method 1: Use a number line.
Check It Out: Example 2A
Find the least common multiple (LCM).
Method 1: Use a number line.
2 and 3
Use a number line to skip count by 2 and 3.
The least common multiple (LCM) of 2 and 3 is 6.

16. Find the least common multiple (LCM). Method 2: Use a list.
Check It Out: Example 2B
Find the least common multiple (LCM).
Method 2: Use a list.
3, 4, and 9
3: 3, 6, 9, 12, 15, 18, 21, 24, 27, 30, 33, 36, . . .
List multiples of 3, 4, and 9.
4: 4, 8, 12, 16, 20, 24, 28, 32, 36, …
Find the smallest number that is in all the lists.
9: 9, 18, 27, 36, 45, . . .
The least common multiple of 3, 4, and 9 is 36.

17. Find the least common multiple (LCM).
Check It Out: Example 2C
Find the least common multiple (LCM).
Method 3: Use prime factorization.
4 and 10
4 = 2 • 2
Write the prime factorization of each number.
10 = • 5
Line up the common factors.
2 • 2 • 5
To find the LCM, multiply one number from each column.
2 • 2 • 5 = 20
LCM: 20

18. Find the least common multiple (LCM).
Check It Out: Example 2D
Find the least common multiple (LCM).
12, 6, and 8
12 = 22 • 3
Write the prime factorization of each number in exponential form.
6 = 2 • 3
8 = 23
To find the LCM, multiply each prime factor once with the greatest exponent used in any of the prime factorizations.
23 • 3
23 • 3 = 24
LCM: 24

19. Lesson Quiz
Find the least common multiple (LCM).
1. 6, , 12
3. 5, 6, , 16, 24, 36
5. Two students in Mrs. Albring’s preschool class are stacking blocks, one on top of the other. Reece’s blocks are 4 cm high and Maddy’s blocks are 9 cm high. How tall will their stacks be when they are the same height for the first time? 42 36 30
144
36 cm