LCM of 4 and 28
The LCM of 4 and 28 is 28. The GCF (HCF) is 4.
Method 1: list the multiples until one matches
| Multiples of | … |
|---|---|
| 4 | 4, 8, 12, 16, 20, 24, 28, 32, … |
| 28 | 28, 56, 84, 112, 140, 168, 196, 224, … |
The first multiple on every list is 28.
Method 2: prime factorisation
4 = 22
28 = 22 × 7
Take every prime that appears in any of them, with its highest power: 22 × 7 = 28.
Method 3: the division (ladder) method
| Divide by | Row |
|---|---|
| 2 | 4 · 28 |
| 2 | 2 · 14 |
| 7 | 1 · 7 |
| 1 · 1 |
Divide the row by a prime that goes into at least one number, bringing the others down unchanged, until every entry is 1. Multiply the divisors: 2 × 2 × 7 = 28.
Method 4: LCM = product ÷ GCF
GCF of 4 and 28 = 4, so LCM = 4 × 28 ÷ 4 = 112 ÷ 4 = 28.
🧮 Calculator
The answer appears here, with the working in words.
✏️ Five questions, answers hidden
1. Find the LCM of 4 and 29.
Answer: 116
2. Find the LCM of 8 and 28.
Answer: 56
3. Add 1/4 + 1/28 (common denominator).
Answer: 8/28
4. Find the GCF of 4 and 28.
Answer: 4
5. Two lights flash every 4 s and 28 s; when do they flash together?
Answer: every 28 s
📚 The class this belongs to
The LCM is taught in Grade 5 and 6 in the US (Common Core 6.NS.4), Year 6 and 7 in the UK, Class 5 and 6 in CBSE and Year 6 and 7 in Australia, usually together with the HCF or GCF.
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