# 4-8 Least Common Multiple

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```					           4-8                Least Common Multiple

MAIN IDEA
Find the least common
Use cubes to build the first row of each prism as shown.
multiple of two or more
1. Add a second row to each prism. Record
numbers.
the total number of cubes used in a table
New Vocabulary                 like the one shown below.
multiple                          Number of Rows         1       2        3        4
least common multiple             Cubes in Prism A       4            8       12       16
(LCM)                            Cubes in Prism B       6           12       18       24

Math Online
glencoe.com                 2. Add rows until each prism has four rows.
• Extra Examples            3. Describe two prisms that have the same
• Personal Tutor
• Self-Check Quiz
number of cubes. See margin.
4. If you keep adding rows, will the two prisms have the same
number of cubes again? See margin.

A multiple is the product of a number and any whole number. The
least common multiple, or LCM, of two or more numbers is the least of
their common multiples, excluding zero.

Find the LCM
1 Find the LCM of 6 and 10.

METHOD 1      List the nonzero multiples.
List the multiples of 6 until you come to a number that is also a
multiple of 10.
multiples of 6: 6, 12, 18, 24, 30, …
multiples of 10: 10, 20, 30, …
Notice that 30 is also a multiple of 10. The LCM of 6 and 10 is 30.

METHOD 2        Use prime factorization.
Multiply               6=2·3                  The prime factors of 6
Everyday Use to find the
10 = 2 · 5              and 10 are 2, 3, and 5.
product
Multiple
Math Use the product of       The LCM is the least product that contains the prime factors of each
a number and any whole        number. So, the LCM of 6 and 10 is 2 · 3 · 5 or 30.
number

Lesson 4-8 Least Common Multiple   211
2 Find the LCM of 45 and 75.
Use Method 2. Find the prime factorization of each number.

45 = 3 · 3 · 5 or 3 2 · 5             The prime factors of 45 and 75
2           are 3 and 5. Write the prime
75 = 3 · 5 · 5 or 3 · 5               factorization using exponents.

The LCM is the product of the prime factors 3 and 5, with each one
raised to the highest power it occurs in either prime factorization.
The LCM of 45 and 75 is 32 · 5 2, which is 225.

Find the LCM of each set of numbers.
a. 3, 12                      b. 10, 12                     c. 25, 30

3 PARTY Ling needs to buy paper plates, napkins, and cups for a
party. Plates come in packages of 12, napkins come in packages of
16, and cups come in packages of 8. What is the least number of
packages she will have to buy if she wants to have the same
number of plates, napkins, and cups?

First find the LCM of 8, 12, and 16.

8 = 2 · 2 · 2 or 2 3
12 = 2 · 2 · 3 or 2 2 · 3             The prime factors of 8, 12, and
4           16 are 2 and 3. Write the prime
16 = 2 · 2 · 2 · 2 or 2               factorization using exponents.

The LCM of 8, 12, and 16 is 2 4 · 3, which is 48.

To find the number of packages of each Ling needs to buy, divide
48 by the amount in each package.

cups: 48 ÷ 8 or 6 packages
plates: 48 ÷ 12 or 4 packages
napkins: 48 ÷ 16 or 3 packages

So, Ling will need to buy 6 packages of cups, 4 packages of plates,
and 3 packages of napkins.

d. VEHICLES Mr. Hernandez changes his car’s oil every 3 months,
rotates the tires every 6 months, and replaces the air filter once a
year. If he completed all three tasks in April, what will be the next
month he again completes all three tasks?

212   Chapter 4 Fractions, Decimals, and Percents
Examples 1–3          Find the LCM of each set of numbers.
(pp. 211–212)
1. 4, 14     28                          2. 6, 7    42                            3. 12, 15   60
4. 21, 35        105                     5. 3, 5, 12       60                     6. 6, 14, 21     42

Example 3        7. GOVERNMENT The number of years per term for a                                        Elected Office Term (yr)
(p. 212)       U.S. President, senator, and representative is shown.                                President                4
Suppose a senator was elected in the presidential
Senator                  6
election year 2008. In what year will he or she
Representative           2
campaign again during a presidential election year?
2020

Find the LCM for each set of numbers.
HOMEWORK         HELP
For           See         8. 6, 8    24                            9. 8, 18     72                         10. 12, 16   48
Exercises     Examples
11. 24, 36        72                     12. 11, 12    132                        13. 45, 63   315
8–13, 20        1, 2
14–19, 21         3         14. 2, 3, 5       30                     15. 6, 8, 9      72                      16. 8, 12, 16     48
17. 12, 15, 28          420              18. 22, 33, 44          132              19. 12, 16, 36     144

Exercise Levels             20. CHORES Hernando walks his dog every
A: 8–21                         two days. He gives his dog a bath once a
B: 22–30                        week. Today, Hernando walked his dog
C: 31–34                        and then gave her a bath. How many
days will pass before he does both                                                Friend   Time Interval
chores on the same day? 14 days                                                  Linda         every 30 min
Brandon       every 45 min
21. TEXT MESSAGING Three friends use text
Edward        every 60 min
messaging to notify their parents of their
whereabouts. If all three contact their parents
at 3:00 p.m., at what time will all three
contact their parents again at the same time?
6:00 P.M.
Find the LCM of each set.
22. \$3.00, \$14.00             \$42        23. 10¢, 25¢, 5¢          50¢            24. 9 inches, 2 feet
72 in. or 6 ft
Write two numbers whose LCM is the given number.
25–28. Sample               25. 35     5, 7                 26. 56   7, 8                  27. 70    10, 35          28. 30      6, 15
29. SNACKS Alvin’s mom needs to buy snacks for soccer practice. Juice boxes
come in packages of 10. Oatmeal snack bars come in packages of 8.
She wants to have the same number of juice boxes and snack bars, what is
the least number of packages of each snack that she will have to buy?
4 packages of juice boxes and 5 packages of oatmeal snack bars
EXTRA   PRACTICE            30. REASONING The LCM of two consecutive positive numbers is greater than
See pages 678, 707.
200 and is a multiple of 7. What are the least possible numbers? 14 and 15

Lesson 4-8 Least Common Multiple               213
H.O.T. Problems              31. CHALLENGE Two numbers have a GCF of 3 · 5. Their LCM is 22 · 3 · 5. If one
of the numbers is 3 · 5, what is the other number? 2 2 · 3 · 5, or 60

32. Number                   32. SELECT A TECHNIQUE The schedule for                       Clark Street Train Station
sense; Sample                   each of three trains is shown. Suppose a
Train                    Leaves Station
answer: The LCM                 train from each line leaves Clark Street
Red-line                every 14 minutes
of 14, 16, and 8 is             at 11:35 a.m. Which of the following
Blue-line               every 16 minutes
112. So, 112                    technique(s) might you use to determine
minutes later, or               the next time all three trains will be leaving Brown-line              every 8 minutes
at 1:27 P.M.                    at the same time? Justify your selection(s).
Then use the technique to solve the problem.

mental math              number sense               estimation

33. OPEN ENDED Write three numbers that have an LCM of 30.
34.   WR ITING IN MATH Describe the relationship between 4, 20, and 5 using
the words factor and multiple. 4 and 5 are factors of 20; 20 is a multiple (and
LCM) of 4 and 5.

35. Which rule describes the common                           36. SHORT RESPONSE Wil swims every
multiples of 12 and 18, where n                              third day, runs every fourth day,
represents the counting numbers? C                           and lifts weights every fifth day.
A 12n                                                        If Wil does all three activities today,
how many days will pass before he
B 18n
does all three activities on the same
C 36n                                                        day again? 60
D 216n

Write each percent as a decimal.          (Lesson 4-7)

37. 55%      0.55             38. 26.4%   0.264          39.   _% 0.0025
1
40. 2%         0.02
4
41. DIAMONDS Sixty-eight percent of engagement rings have a diamond that is
round in shape. Write this percent as a fraction in simplest form.              (Lesson 4-6)    _
17
25
42. ALGEBRA Solve 3x = 18. (Lesson 3-3)        6

43. ALGEBRA Rose swam 7 laps more than twice the number of laps her sister
swam. Write an algebraic expression to represent this situation.               (Lesson 3-1)    2s + 7

PREREQUISITE SKILL Replace each ● with <, > or = to make a true sentence.                          (Page 670)

44. 6.85 ● 5.68       >               45. 2.34 ● 2.43    <                 46. 6.9 ● 5.99    >

214    Chapter 4 Fractions, Decimals, and Percents

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