Problem
Reach the next hour, then add the rest
Mia started reading at 1:50 p.m. She read a storybook for 35 minutes, and then read a science book for 47 minutes. At what time (p.m.) did Mia finish reading?
Measurement & data
Your answer
Watch out If you add 35 and 47 into 82 minutes and just stack it onto 1:50, you get 50+82=132 and it's easy to lose track of how many times 60 minutes rolls over into an hour. Make the next o'clock first, then add what's left, so each 60-minute jump carries exactly one hour.
How to solve
1STEP 1
Reach the next o'clock
From 1:50, it takes 10 minutes to reach 2:00.
60 − 50 = 10, 1:50 + 10 = 2:00
Adding 10 minutes to 50 makes 60 minutes, and 60 minutes is exactly one hour, so the clock rolls over to the next o'clock. Spending just those 10 minutes lands exactly on 2:00.
Identify Subproblems2.MD.C.72STEP 2
Add the rest of the storybook time
With 10 of the 35 minutes used, 25 minutes remain, reaching 2:25.
35 − 10 = 25, 2:00 + 25 = 2:25
Splitting 35 minutes into 10 and 25 and adding them one after another lands on exactly the same time as adding all 35 at once — the two pieces make up the full 35 with nothing left out or doubled, and the order of adding never changes the total.
Identify Subproblems3.MD.A.1Reading 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
From 2:25 to 3:00 is 35 minutes, then 12 more reaches 3:12.
2:25 + 35 = 3:00, 47 − 35 = 12, 3:00 + 12 = 3:12
The 47 minutes split the same way: 35 minutes fill the gap from 2:25 to 3:00, and the leftover 12 minutes come right after, landing at exactly the time adding all 47 minutes at once would give — 3:12.
Identify Subproblems3.MD.A.1Answer
3:12 p.m.
1:50 + 35 + 47 = 3:12
Check — total reading time is 35+47=82 minutes (1 h 22 min). Adding 1 hour to 1:50 gives 2:50, then 22 more minutes gives 3:12 — the same answer.
Takeaway
When an addition awkwardly crosses an hour boundary, first make the next o'clock, then add what's left — no confusion.
- 1:50 + 10 = 2:00
- +25 more reaches 2:25
- Repeat for 47 minutes to reach 3:12
Where next?
Level upsuggested
Another one like thissuggested
Step back and solidifysuggested