Question
Find Least Common Multiple of 13, 15, 18 and 21 using prime factorisation method.
Answer
LCM( 13, 15, 18, 21 ) = 8190
Explanation
Step 1: Write down factorisation of each number:
13 = 13
15 = 3 · 5
18 = 2 · 3 · 3
21 = 3 · 7
Step 2 : Match primes vertically:
| 13 | = | 13 | ||||||||||
| 15 | = | 3 | · | 5 | ||||||||
| 18 | = | 2 | · | 3 | · | 3 | ||||||
| 21 | = | 3 | · | 7 |
Step 3 : Bring down numbers in each column and multiply to get LCM:
| 13 | = | 13 | ||||||||||||
| 15 | = | 3 | · | 5 | ||||||||||
| 18 | = | 2 | · | 3 | · | 3 | ||||||||
| 21 | = | 3 | · | 7 | ||||||||||
| LCM | = | 2 | · | 3 | · | 3 | · | 5 | · | 7 | · | 13 | = | 8190 |
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