Answer
140
Work Step by Step
First, we express each number as a product of prime factors:
$20=2\times2\times5$
$28=2\times2\times7$
The least common multiple includes each different prime factor as many times as the most times it appears in one of the factorizations.
The most times 2 appears is twice.
The most times 5 appears is once.
The most times 7 appears is once.
Therefore,
LCM$=2\times2\times5\times7=140$
