Measure Elapsed Time

It is 7:30. The hour hand tells you that it is after 7 o’clock, and not yet 8 o’clock. The minute hand tells you it is 30 minutes after 7:00, so it is 7:30.

Each number on the clock face stands for the hour of the day, but it also stands for 5-minute intervals. You can count by 5s to find how many minutes it is after an hour.

You can add the minutes. Even put the sandwich in at 30 minutes past 7. 10 more than 30 is 40. Evan takes his sandwich out of the oven at 7:40.

It was 4:00 p.m. You can subtract 1 hour from the hours of the time. 5 – 1 = 4.

60 minutes pass between 4:00 and 5:00 because it is one hour, and one hour is the same amount of time as 60 minutes. Half an hour passes between 5:00 and 5:30, because 30 minutes is the same amount of time as half an hour.

Noon is the same as 12:00 p.m., so I can use a number line and hop 2 hours from 9:30 to 11:30, then another half hour to noon, for a total of 2 ½ hours.

Use a number line. Given the start time, you hop forward to add the elapsed time and find the end time. 4:00 plus half an hour is 4:30 p.m. Given the end time, you hop backward to subtract the elapsed time and find the start time. 5:00 minus half an hour is 4:30 p.m.

Jason must leave before the start of his lesson, so I need to hop backward on the number line to find out what time he should leave: 1:00 p.m.

You could jump one hour, then jump in 5-minute intervals, like on a clock, to find the end time. Or, you could jump two hours, then jump back 20 minutes, because 40 minutes is 20 minutes less than one hour.

Kari is not correct. Start at 7:40 and count forward 37 minutes on a number line to find 8:17 p.m. This is after Kari’s class starts, so she will be late.