SME
Grade 3 Math · Problem type
Find the start time
Use this for “It ended at 4:20 after 35 minutes. When did it start?”
Diagram
Start 3:45, end 4:20: 35 minutes.
This is an example. Type your own values to start a new problem.
Start hour
:
Start minutes
+
Minutes it takes
minutes
=
End hour
:
End minutes
Formulas
45 minutes + 35 minutes ends at 20 minutes past the hour
Start minutes + minutes it takes = end minutes, past the hour
3:45 + 35 minutes = 4:20
Start hour:start minutes + minutes it takes = end hour:end minutes
Assumptions
•
Count back from the end: to the hour, then whole hours, then the minutes left.
•
After 12:59 the clock starts again at 1:00.
•
60 minutes make 1 hour.
Step-by-step
WE KNOW
End hour: 4
End minutes: 20
Minutes it takes: 35
FIND
start minutes, start hour
Step 1 · Find start minutes
? minutes + 35 minutes ends at 20 minutes past the hour
Start minutes + minutes it takes = end minutes, past the hour
Count back the minutes from the end minutes. Past the hour, go back into the hour before.
Count back 35 from 20: back 20 to the hour, then 15 more: 45
Start minutes = 45
Step 2 · Find start hour
?:45 + 35 minutes = 4:20
Start hour:start minutes + minutes it takes = end hour:end minutes
Count back from the end time: to the hour, then whole hours, then the rest.
4:20 → 4:00 is 20 minutes
4:00 → 3:45 is 15 minutes
20 + 15 = 35 minutes
Start hour = 3 (3:45)
Check
Put the numbers back in. Both sides should match.
45 minutes + 35 minutes ends at 20 minutes past the hour ✓
3:45 + 35 minutes = 4:20 ✓
Topic check
See what you know: 5 questions on this topic
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