The GCF of 15 and 18 is 3. The LCM of 15 and 18 is 90.
Factors of 15: 1, 3, 5, 15.
Factors of 18: 1, 2, 3, 6, 9, 18.
Common factors: 1 and 3. The greatest of these is 3.
15 = 3 × 5
18 = 2 × 3 × 3
GCF: multiply the prime factors both numbers share: 3 = 3.
LCM: multiply every prime the maximum number of times it appears in either number: 2 × 3 × 3 × 5 = 90.
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, 18) = 3.
Multiples of 15: 15, 30, 45, 60, 75, 90, 105, ...
Multiples of 18: 18, 36, 54, 72, 90, 108, ...
The first number in both lists is 90.
3 × 90 = 270 = 15 × 18. This always holds for two numbers, so you can find the LCM as (15 × 18) ÷ 3 = 90.
Divide the top and bottom by the GCF 3: 15/18 = 5/6.
| 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 18 | 2 | 18 |
| 3 and 18 | 3 | 18 |
| 4 and 18 | 2 | 36 |
| 5 and 18 | 1 | 90 |
| 6 and 18 | 6 | 18 |
| 7 and 18 | 1 | 126 |
See also: factors of 15 · factors of 18 · 15/18 as a decimal.
Different numbers? Use the GCF and LCM calculator (any amount of numbers) or browse all GCF and LCM pairs.