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Plz tell me this sentences are correct or not?

I'm learning English in the U.S. so I wonder these sentences I made are wrong or not. Plz review and correct them. 1) A. Are all of the teachers from the U.S.? B. Are all of the teachers in this school native speaker? 2) In front our building, there was an accident. Some gasoline gas was leaking. 3) A.(In a post office) When can this letter be reached? -(when i used this sentence in a post office, a staff who work in the post office didn't understand my question) B. (so i thought my question again when i get back home) How long will it take to get there? 4)Look at the page 137 real quick 5)I drank at/in my friend's house last night. (which one is correct between at and in?) 6)Yesterday, you got drunk/drunken? (which one is correct? drunk or drunken?) 7)How many Chinese student are there in one class? 8)A.What was (original/standard) price before I get a discount? B.How much did I get a discount? or How much percentage did I get a discount? 9) What this coupon for? (As showing a coupon to clerk cuz i don't know where I should how I should this coupon)

Public Comments

1. 1. are all the teachers from the U.S? are all the teachers in this school native speakers? 5. in 6. drunk 7. how many Chinese students are there in one class? 8. original how much discount did i get? 9. what is this coupon for?