# What is the Least Common Multiple of 21 and 40?

Least common multiple or lowest common denominator (lcd) can be calculated in two way; with the LCM formula calculation of greatest common factor (GCF), or multiplying the prime factors with the highest exponent factor.

Least common multiple (LCM) of 21 and 40 is 840.

LCM(21,40) = 840

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## Least Common Multiple of 21 and 40 with GCF Formula

The formula of LCM is LCM(a,b) = ( a × b) / GCF(a,b).
We need to calculate greatest common factor 21 and 40, than apply into the LCM equation.

GCF(21,40) = 1
LCM(21,40) = ( 21 × 40) / 1
LCM(21,40) = 840 / 1
LCM(21,40) = 840

## Least Common Multiple (LCM) of 21 and 40 with Primes

Least common multiple can be found by multiplying the highest exponent prime factors of 21 and 40. First we will calculate the prime factors of 21 and 40.

### Prime Factorization of 21

Prime factors of 21 are 3, 7. Prime factorization of 21 in exponential form is:

21 = 31 × 71

### Prime Factorization of 40

Prime factors of 40 are 2, 5. Prime factorization of 40 in exponential form is:

40 = 23 × 51

Now multiplying the highest exponent prime factors to calculate the LCM of 21 and 40.

LCM(21,40) = 31 × 71 × 23 × 51
LCM(21,40) = 840