X, Y, and Z Vs. X, Y and Z

[praveenkvma]

I am not sure about the use of comma before "and". Sometimes, I see examples like X, Y, and Z and sometimes X, Y and Z

Please let me know when to use what?

[Ben Ku]

on the GMAT, there will be a comma before the "and" that lists the last item. For example:

Julie traveled to many countries in South America, such as Paraguay, Argentina, and Peru.

Usually if there is no comma before the "and," the items before and after the "and" are part of one item in the list.

He could not decide whether to select mashed potatoes, macaroni and cheese, or steamed vegetables for his side dish.