Difference between these cron commands

Hi all I want to make sure I was understanding this correctly

if a cron job command was

* */20 * * * command

does that mean this command will run every 20 hours?

also what is the difference between the following two?

0,20,40 * * * * command
20 * * * * command 

I believe the first one runs the command at 0, 20 and 40 minutes while the other runs every 20 minutes. . but I am not sure. Thanks:b:

sorry bad post

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First cron: Hourly except at 20:00 . What did you intend? I'm assuming that this certain editions of Linux because the syntax is invalid in unix.

Second cron: every 20 mins (On the hour; twenty minutes past the hour; and forty minutes past the hour).

Third cron: At 20 minutes past every hour.

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I'm just learning syntax differences. Thank you for your reply I appreciate it!