Question
Find Least Common Multiple of 20, 30, 60 and 75 using prime factorisation method.
Answer
LCM( 20, 30, 60, 75 ) = 300
Explanation
Step 1: Write down factorisation of each number:
20 = 2 · 2 · 5
30 = 2 · 3 · 5
60 = 2 · 2 · 3 · 5
75 = 3 · 5 · 5
Step 2 : Match primes vertically:
| 20 | = | 2 | · | 2 | · | 5 | ||||
| 30 | = | 2 | · | 3 | · | 5 | ||||
| 60 | = | 2 | · | 2 | · | 3 | · | 5 | ||
| 75 | = | 3 | · | 5 | · | 5 |
Step 3 : Bring down numbers in each column and multiply to get LCM:
| 20 | = | 2 | · | 2 | · | 5 | ||||||
| 30 | = | 2 | · | 3 | · | 5 | ||||||
| 60 | = | 2 | · | 2 | · | 3 | · | 5 | ||||
| 75 | = | 3 | · | 5 | · | 5 | ||||||
| LCM | = | 2 | · | 2 | · | 3 | · | 5 | · | 5 | = | 300 |
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