now, she writes 4 notes every 1 hour.
so in 2 hours she has written 8 notes, and the remaining is 50 - 8 or 42.
in 3 hours has written 12 notes, and remaining are 50 - 12, or 38, and so on.
now, how many hours will it take her to them all? or, how many times does 4 go into 50? 50 /4 or 12.5.
so it will take her 12 hours and a half to do them all, at that time, the remaining notes are 0, because she's done, and the hours are 12 and a half.