The GCF of 20 and 28 is 4. The LCM of 20 and 28 is 140.
Factors of 20: 1, 2, 4, 5, 10, 20.
Factors of 28: 1, 2, 4, 7, 14, 28.
Common factors: 1, 2 and 4. The greatest of these is 4.
20 = 2 × 2 × 5
28 = 2 × 2 × 7
GCF: multiply the prime factors both numbers share: 2 × 2 = 4.
LCM: multiply every prime the maximum number of times it appears in either number: 2 × 2 × 5 × 7 = 140.
Divide the larger number by the smaller and repeat with the remainder until it is 0. The last non-zero remainder is the GCF.
The last non-zero remainder is 4, so GCF(20, 28) = 4.
Multiples of 20: 20, 40, 60, 80, 100, 120, 140, 160, ...
Multiples of 28: 28, 56, 84, 112, 140, 168, ...
The first number in both lists is 140.
4 × 140 = 560 = 20 × 28. This always holds for two numbers, so you can find the LCM as (20 × 28) ÷ 4 = 140.
Divide the top and bottom by the GCF 4: 20/28 = 5/7.
| Numbers | GCF | LCM |
|---|---|---|
| 2 and 20 | 2 | 20 |
| 3 and 20 | 1 | 60 |
| 4 and 20 | 4 | 20 |
| 5 and 20 | 5 | 20 |
| 6 and 20 | 2 | 60 |
| 7 and 20 | 1 | 140 |
| 2 and 28 | 2 | 28 |
| 3 and 28 | 1 | 84 |
| 4 and 28 | 4 | 28 |
| 5 and 28 | 1 | 140 |
| 6 and 28 | 2 | 84 |
| 7 and 28 | 7 | 28 |
See also: factors of 20 · factors of 28.
Different numbers? Use the GCF and LCM calculator (any amount of numbers) or browse all GCF and LCM pairs.