basic concept 要教清楚,1 hour=60mins, after 1 hour 60 mins, it goes to 2 hour 60 mins(ie, 3 hour) and then 3 hour 60 mins(ie 4 hour).... etc. You can call it clock math which is different from normal math which keeps going up 1,2,3,4...million....until infinity but for clock math there is limit, after 60 mins, its 1 hour and so on