Finding a Past Time
Grade 4 students master finding a past time by counting backward from a known end time in Saxon Math Intermediate 4. The method subtracts hours first, then minutes — for example, a movie ending at 9:05 p.m. that ran 2 hours 15 minutes started at 6:50 p.m. When subtracting minutes that cross the hour mark, students borrow 60 minutes from the hours column. This Chapter 3 skill builds number sense around elapsed time and prepares students for multi-step time problems common on standardized tests.
Key Concepts
Property To find a time that occurred in the past, you must count backward from a known, later time.
Examples What time was it 3 hours and 10 minutes before 1:30 p.m.? Step 1: 1:30 p.m. 3 hours = 10:30 a.m. Step 2: 10:30 a.m. 10 minutes = 10:20 a.m. When did the movie start if it ended at 9:05 p.m. and was 2 hours 15 minutes long? Step 1: 9:05 p.m. 2 hours = 7:05 p.m. Step 2: 7:05 p.m. 15 minutes = 6:50 p.m.
Explanation Imagine you know a big event happened today, but you need to figure out when it actually started. This is like being a time detective and rewinding the clock! You start at the end time and travel backward. Just like with going forward, it is easiest to take big leaps backward with the hours first, and then carefully step back by the minutes.
Common Questions
How do you find a past time in math?
Start with the known end time, subtract the hours first to make the biggest jump backward, then subtract the remaining minutes. If the minutes require borrowing, take 60 minutes from the current hour before subtracting.
How do you subtract time when minutes cross the hour?
If you cannot subtract the minutes directly, borrow one hour (60 minutes) from the hours column. Add those 60 minutes to the current minutes, then subtract. For example, subtracting 30 minutes from 2:15 p.m. means going to 2:00 p.m. first (15 minutes), then back 15 more minutes to 1:45 p.m.
What is an example of finding a start time?
A basketball game ended at 4:15 p.m. and lasted 2 hours 30 minutes. Subtract 2 hours: 4:15 becomes 2:15 p.m. Then subtract 30 minutes, borrowing from the hour: 2:15 becomes 1:45 p.m. The game started at 1:45 p.m.
Why subtract hours before minutes in elapsed time problems?
Subtracting hours first makes the largest jump backward and simplifies the remaining minutes subtraction. It avoids confusion from trying to handle both units at the same time.
What grade level covers finding a past time?
Finding a past time is a Grade 4 skill covered in Saxon Math Intermediate 4, Chapter 3 (Lessons 21–30). It is a foundational elapsed time concept on state math assessments.