Time: adding and subtracting named numbers
Change larger units of time into smaller ones, add or subtract, and write the answer in two units. Find how long an event lasted from its start and end.
- Ukraine: grade 4
- Russia: grade 4
Guide
Units of time: 1 day = 24 hours, 1 week = 7 days, 1 year = 12 months, 1 century = 100 years, 1 hour = 60 minutes, 1 minute = 60 seconds. A compound number like 7 h 32 min has two units; 452 min has one.
Add and subtract in a column: minutes under minutes, hours under hours. If the minutes pass 60, change 60 min into 1 h. If you cannot subtract, borrow a larger unit: 7 h 32 min = 6 h 92 min.
Example: 7 h 32 min - 5 h 45 min = 6 h 92 min - 5 h 45 min = 1 h 47 min. Check: 1 h 47 min + 5 h 45 min = 6 h 92 min = 7 h 32 min.
A time problem goes in a table: start of the event, duration, end of the event. Before 12 August 11 whole days have passed, and at 11 am 11 whole hours: 11 days 11 h. In the same way 16 August 7 pm is 15 days 19 h.