LCM of 20 and 30 = 60
Quick Answer
LCM(20 and 30) = 60.
First common multiples: 60, 120, 180, 240, 300. Related GCF: 10.
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Step-by-Step: LCM via the GCF Identity
The fastest way to compute the LCM uses the identity LCM(a, b) × GCF(a, b) = a × b. So LCM = (a × b) / GCF. We first find the GCF with the Euclidean algorithm, then apply the identity.
Euclidean reduction for (20, 30):
| Step | Dividend ÷ Divisor | Quotient | Remainder |
|---|---|---|---|
| 1 | 30 ÷ 20 | 1 | 10 |
| 2 | 20 ÷ 10 | 2 | 0 |
GCF = 10. Applying the identity:
LCM = (20 × 30) ÷ GCF = 600 ÷ 10 = 60
LCM(20 and 30) = 60
First 5 Common Multiples of 20 and 30
Every common multiple is a multiple of the LCM. The first five are:
| k | k × LCM | Value |
|---|---|---|
| 1 | 1 × 60 | 60 |
| 2 | 2 × 60 | 120 |
| 3 | 3 × 60 | 180 |
| 4 | 4 × 60 | 240 |
| 5 | 5 × 60 | 300 |
Related: GCF of 20 and 30
GCF(20, 30) = 10
From the identity LCM × GCF = a × b:
60 × 10 = 600 = 20 × 30
See the dedicated GCF of 20 and 30 page for the full common-divisor list and Euclidean walk-through.
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How to Find the Least Common Multiple
The Least Common Multiple (LCM) of two or more positive integers is the smallest positive integer divisible by each of them.
Method 1: GCF Identity (used above)
- Find the GCF (Greatest Common Factor) using the Euclidean algorithm.
- Apply the identity: LCM = (a × b) / GCF.
Method 2: Prime Factorization
- Find the prime factorization of each number.
- For each prime that appears in any factorization, take the highest exponent across all inputs.
- Multiply these prime powers together.
Example: LCM(12, 18) via prime factorization
- 12 = 2² × 3¹
- 18 = 2¹ × 3²
- Take the highest exponent of each prime: 2² (from 12), 3² (from 18).
- LCM = 2² × 3² = 4 × 9 = 36
Method 3: Listing Multiples
- List the multiples of each number until you find a common one.
- That smallest common multiple is the LCM.
This is the slowest method for large inputs; the GCF identity is preferred.
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Frequently Asked Questions
What is the LCM of 20 and 30?
LCM(20 and 30) = 60. This is the smallest positive integer divisible by each input.
How is the LCM of 20 and 30 calculated?
Apply the identity LCM = (a × b) / GCF. For 20 and 30, GCF = 10, so LCM = 600 / 10 = 60.
How are LCM and GCF related?
By the identity LCM(a, b) × GCF(a, b) = a × b. So once you have the GCF (via the Euclidean algorithm), the LCM follows immediately: LCM = (a × b) / GCF.
What is LCM used for?
Two major uses: (1) adding/subtracting fractions with different denominators (find the common denominator); (2) scheduling — when two repeating events occur together (every 12 days and every 18 days → coincide every LCM(12, 18) = 36 days).
Related Calculators
- GCF of 20 and 30 = 10
- Prime Factorization Calculator
- Divisors of a Number
- Multiples of a Number
- Fraction Calculator (uses LCM for common denominators)
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