Time / Add & Subtract Time
Add & Subtract Time
Add and subtract hours and minutes — including carrying and borrowing across the 60-minute mark. Grades 3–5.
The Key Rule: Time is Base-60
Unlike regular addition (base-10), time uses base-60 for minutes and seconds. 60 minutes = 1 hour. When minutes exceed 59, carry 1 to hours. When borrowing, take 1 hour = 60 minutes.
➕ Adding Time
2 hr 45 min
+ 1 hr 30 min
3 hr 75 min
= 4 hr 15 min (75 min = 1h 15m)
If minutes ≥ 60: subtract 60 from minutes and add 1 to hours.
➖ Subtracting Time
4 hr 20 min
- 1 hr 45 min
Can't do 20 − 45, so borrow:
3 hr 80 min
- 1 hr 45 min
= 2 hr 35 min
If minutes not enough: borrow 1 hour (60 minutes) from the hour column.
Working With AM & PM
- •AM = before noon (midnight to 11:59am)
- •PM = afternoon/evening (noon to 11:59pm)
- •12:00 noon is 12:00 PM; 12:00 midnight is 12:00 AM (confusing — teach explicitly)
- •When adding time that crosses noon, flip AM → PM at 12:00
- •Example: 10:30 AM + 3 hr 45 min = 14:15 = 2:15 PM
Practice Drills
20
Example: 6 × 1