Lesson 7: Adding and Subtracting Big Numbers
Questions to Explore
- What strategies can we use to add and subtract?
- How does estimation help with problem solving?
Facts and Definitions
- Estimate: to guess
- Fluently add and subtract multi-digit whole numbers using the standard algorithm
- Use place value understanding to round multi-digit whole numbers to any place
- fine point dry-erase markers (kit)
- whiteboard (kit)
Materials: fine point dry-erase markers (kit), whiteboard (kit)
During this lesson, your child will review adding and subtracting numbers to 1,000 and will begin to add and subtract even larger numbers. As needed, allow him to refer to the "Working With Big Numbers" sheet that shows examples of composing 10 when adding and examples of borrowing when subtracting. He can store this sheet in his Interactive Notebook for future reference.
Ask your child to write and solve the following problems on the whiteboard (he should write them vertically):
- 546+378 (924)
- 724−198 (526)
When he's found the answers to the given problems, ask him to explain how he did so. He should talk about composing 10 and carrying when adding and borrowing when subtracting. If needed, give him the following problems for additional practice:
- 199+98 (297)
- 487+318 (805)
- 825−651 (174)
- 636−473 (163)
Next, ask your child to write 637+748 on the whiteboard, and ask, "Will the sum of these numbers be greater than or less than 1000?" and "How do you know?" Help your child see that he can round to estimate the answer. For example, if he rounds the numbers to the nearest 100 he should end up with 600+700 for an estimated sum of 1300. Now, show him how to complete the original addition problem by carrying the 1 from adding the digits in the hundreds places (6+7=13) over the "invisible" thousands and then bringing the 1 down to the thousands place under the line to finish the sum.
Say, "Once you know how to line up your numbers correctly and carry to the next place, you can add really big numbers, even numbers in the millions."