The 'of' construction sounds slightly irregular to me, although I can't tell you exactly why. The others are fine. ]
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TuongvanCan I say "How many years' experience do you have of / in teaching English ?Looks fine to me.
Best wishes
TuongvanI have over 20 years' experience teaching English?This is an interesting sentence. I just found a similar one from one of my books and I am not quite sure how to understand it.