First add the seconds you have 45 +5+10 seconds which equals 60 seconds or 1 minutes. Now add the minutes 9 + 7 + 12 minutes = 28 minutes then add that 1 minutes that you got from the seconds 28 + 1 minute = 29 minutes 7:40 + 20 minutes = 8:00 + 9 minutes = 8:09 am. So Marsha arrived to school at 8:09 am. Comment if you have any questions.