Strategies for Adding Time
This Grade 4 Eureka Math skill teaches three strategies for adding mixed units of time. Students can combine like units (adding hours and minutes separately), use decomposition to make the next whole hour, or use compensation by adjusting one addend to simplify the calculation. For example, 2 hours 40 minutes + 1 hour 35 minutes can be solved as (3 hours 75 minutes = 4 hours 15 minutes) using combining, or by making a full hour from the minutes. All strategies yield the same correct answer. These flexible approaches appear in Chapter 35 of Eureka Math Grade 4.
Key Concepts
Property To add mixed units of time, you can use different strategies such as combining like units, making the next whole unit (decomposition), or using compensation. All valid strategies will lead to the same correct answer.
Examples Problem: Find the sum of $2$ hours $40$ minutes and $1$ hour $35$ minutes. Combining Like Units: Add hours and minutes separately: $(2+1)$ hours $(40+35)$ minutes = $3$ hours $75$ minutes. Since $75$ min = $1$ hr $15$ min, the total is $4$ hours $15$ minutes. Making the Next Whole Unit: Take $20$ minutes from $1$ hr $35$ min to complete the hour with $2$ hr $40$ min. This makes $3$ hours. The remaining time is $1$ hr $15$ min. Adding them gives $3$ hours + $1$ hour $15$ minutes = $4$ hours $15$ minutes. Compensation: Round $2$ hr $40$ min up to $3$ hours (by adding $20$ min). Add $3$ hours + $1$ hour $35$ minutes = $4$ hours $35$ minutes. Then, subtract the $20$ minutes you added: $4$ hr $35$ min $20$ min = $4$ hours $15$ minutes.
Explanation There are several effective methods for adding mixed units of time. You can combine the hours and minutes separately and then regroup, which is similar to the standard addition algorithm. Alternatively, you can use mental math strategies like decomposition (making a whole unit) or compensation (rounding and adjusting). Choosing the best strategy often depends on the specific numbers in the problem and your personal preference.
Common Questions
What are the three strategies for adding mixed units of time?
Combining like units (add hours, add minutes, regroup if needed), making the next whole hour by decomposing minutes, and compensation by adjusting one addend to a round number.
How do you add 2 hours 40 minutes and 1 hour 35 minutes by combining like units?
Add hours: 2+1=3. Add minutes: 40+35=75. Since 75 minutes = 1 hour 15 minutes, regroup: 3 hours + 1 hour 15 minutes = 4 hours 15 minutes.
How do you use the make-a-whole-hour strategy for this problem?
Take 20 minutes from 35 minutes to complete 2 hours 40 minutes to 3 hours. Remaining: 1 hour 15 minutes. Sum: 3 hours + 1 hour 15 minutes = 4 hours 15 minutes.
What is the compensation strategy for adding time?
Adjust one addend to a round number, add, then undo the adjustment. For example, round 2 hours 40 min up to 3 hours (adding 20 min), add: 3 hours + 1 hour 35 min = 4 hours 35 min, then subtract 20 min: 4 hours 15 min.
When do you need to regroup minutes into hours?
When the sum of the minutes is 60 or more, you regroup: every 60 minutes becomes 1 additional hour, and the remainder stays as minutes.