The greatest common factor is 4.
Multiply 16 * 20.
Divide that answer by 4.
This site shows that method plus another similar method.
http://www.aaamath.com/fra66i-lcm.html
WHAT ARE THE LCM OF 16 AND 20
2 answers
...16 20
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4 | 4 5
4 | 1 5
5 | 1 1
so LCM = 4x4x5 = 80
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4 | 4 5
4 | 1 5
5 | 1 1
so LCM = 4x4x5 = 80