The GCF of 20 and 40 is 20. The LCM of 20 and 40 is 40.
Factors of 20: 1, 2, 4, 5, 10, 20.
Factors of 40: 1, 2, 4, 5, 8, 10, 20, 40.
Common factors: 1, 2, 4, 5, 10 and 20. The greatest of these is 20.
20 = 2 × 2 × 5
40 = 2 × 2 × 2 × 5
GCF: multiply the prime factors both numbers share: 2 × 2 × 5 = 20.
LCM: multiply every prime the maximum number of times it appears in either number: 2 × 2 × 2 × 5 = 40.
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 20, so GCF(20, 40) = 20.
Multiples of 20: 20, 40, 60, ...
Multiples of 40: 40, 80, ...
The first number in both lists is 40.
20 × 40 = 800 = 20 × 40. This always holds for two numbers, so you can find the LCM as (20 × 40) ÷ 20 = 40.
Divide the top and bottom by the GCF 20: 20/40 = 1/2.
| 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 40 | 2 | 40 |
| 3 and 40 | 1 | 120 |
| 4 and 40 | 4 | 40 |
| 5 and 40 | 5 | 40 |
| 6 and 40 | 2 | 120 |
| 7 and 40 | 1 | 280 |
See also: factors of 20 · factors of 40.
Different numbers? Use the GCF and LCM calculator (any amount of numbers) or browse all GCF and LCM pairs.