The GCF of 15 and 21 is 3. The LCM of 15 and 21 is 105.
Factors of 15: 1, 3, 5, 15.
Factors of 21: 1, 3, 7, 21.
Common factors: 1 and 3. The greatest of these is 3.
15 = 3 × 5
21 = 3 × 7
GCF: multiply the prime factors both numbers share: 3 = 3.
LCM: multiply every prime the maximum number of times it appears in either number: 3 × 5 × 7 = 105.
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 3, so GCF(15, 21) = 3.
Multiples of 15: 15, 30, 45, 60, 75, 90, 105, 120, ...
Multiples of 21: 21, 42, 63, 84, 105, 126, ...
The first number in both lists is 105.
3 × 105 = 315 = 15 × 21. This always holds for two numbers, so you can find the LCM as (15 × 21) ÷ 3 = 105.
Divide the top and bottom by the GCF 3: 15/21 = 5/7.
| Numbers | GCF | LCM |
|---|---|---|
| 2 and 15 | 1 | 30 |
| 3 and 15 | 3 | 15 |
| 4 and 15 | 1 | 60 |
| 5 and 15 | 5 | 15 |
| 6 and 15 | 3 | 30 |
| 7 and 15 | 1 | 105 |
| 2 and 21 | 1 | 42 |
| 3 and 21 | 3 | 21 |
| 4 and 21 | 1 | 84 |
| 5 and 21 | 1 | 105 |
| 6 and 21 | 3 | 42 |
| 7 and 21 | 7 | 21 |
See also: factors of 15 · factors of 21.
Different numbers? Use the GCF and LCM calculator (any amount of numbers) or browse all GCF and LCM pairs.