Answer
$84$
Work Step by Step
The least common multiple ($LCM$) of two numbers $a$ and $b$ is the smallest number which is a multiple of both numbers $a$ and $b$.
We have to determine the $LCM$ of the numbers $21$ and $28$.
First we write the prime factorization of each number:
$21=3\cdot 7$
$28=2\cdot 2\cdot 7$
Then we take each factor the greatest number of times that it appears in any one prime factorization:
$LCM=2\cdot 2\cdot 3\cdot 7 = 84$.
The $LCM$ of the given numbers is $84$.
