Problem
Reach the next hour, then add the rest
Mia started reading at 1:50 p.m. She read a storybook for 35 minutes, then a science book for 47 minutes. Find the time she finished.
Measurement & data
Your answer
Watch out The question asks for the finishing time after both books, not just after the storybook.
How to solve
Strategy Identify Subproblems — Rather than adding 35 and 47 across an awkward minute boundary, first add just enough minutes to reach the next whole hour (2:00), then add the rest. This 'make o'clock then add' approach keeps each addition simple.
1STEP 1
Reach the next o'clock
From 1:50 it takes 10 minutes to reach 2:00.
60 - 50 = 10 minutes → 1{:}50 + 10 = 2{:}00
Topping the minutes up to 60 lands exactly on the next whole hour, which is easy to add onto.
2.MD.C.7Identify Subproblems2STEP 2
Add the rest of the storybook time
The storybook was 35 minutes; 10 reached 2:00, so 25 minutes remain.
35 - 10 = 25 → 2{:}00 + 25 = 2{:}25
After hitting the o'clock, adding the leftover minutes is a simple within-the-hour jump.
3.MD.A.1Identify SubproblemsReading 35 minutes starting at 1:50 p.m. ends at 2:25, reached by first spending 10 minutes to hit 2:00 and then 25 minutes past it.
Why?
Splitting the 35-minute jump into 10 minutes up to 2:00 and 25 minutes after it, then taking those jumps one right after the other, lands on the very same time as adding 35 minutes all at once.
Why?
The two pieces, 10 minutes and 25 minutes, together make up exactly the whole 35 minutes, with nothing left out and nothing counted twice.
🧱Whole is the sum of its partsBreak a thing into pieces with no gaps or overlaps and the pieces add back to the whole.
Why?
Adding 10 minutes and then 25 minutes gives the same total as adding the full 35 at once, since it never matters how you group the minutes you add.
🧱Associativity of additionAdding three piles, it does not matter which two you push together first.
Why?
The first 10 minutes reach exactly 2:00 because 50 minutes plus 10 minutes make 60 minutes, and 60 minutes always equal one whole hour, so the clock rolls over to the next o'clock.
🧱A bigger unit is a fixed count of smaller unitsA bigger unit always equals the same fixed number of smaller units (1 hour = 60 minutes, 1 km = 1000 m), so a measure can be renamed between them.
3STEP 3
Add the science book time
Add 47 minutes to 2:25: reach 3:00 (35 min), then 12 more lands on 3:12.
2{:}25 + 35 = 3{:}00, 47 - 35 = 12 → 3{:}00 + 12 = 3{:}12
Using the same make-o'clock trick again keeps the second addition simple.
3.MD.A.1Identify SubproblemsAnswer
3:12 p.m.
3:00 + 12 = 3:12 p.m.
Total reading time is 35 + 47 = 82 minutes = 1 hour 22 minutes. Starting at 1:50, add 1 hour to get 2:50, then 22 minutes to get 3:12 p.m. — matching the step-by-step result.
Takeaway
Bump up to the next o'clock first, then add what's left: a Grade 3 time-adding shortcut!
- Reach the next o'clock
- Add the rest of the storybook time
- Add the science book time
Where next?
Level upsuggested
Another one like thissuggested
Step back and solidifysuggested