He arrives at 9:30 because if you take 8:40 (when dealing with time I like to think of it as a whole number so 8:40 would become 840) then just add 50 minutes to 8:40 (or 840+50 which equals 890 also know as 9:30 because you have to subtract 60 from 90 (because 60 is the limit in time (well technically 59 is)) then add one to 8 like so, which equals 9:30 Enjoy!=)
