Explain to students that when estimating the difference you start by rounding the numbers in the problem to make an easier problem and then solving the rounded equation. The problem 49,183 - 10,722 ≈ can be solved using the estimated problem 50,000 - 11,000. Then use another example to explain that you can round the numbers to the nearest thousand or ten thousands number. Ask students to round each number and ask students to explain their strategy. Emphasize that, while estimating, there are different possible solutions. Practice matching the next three problems to their best estimated problems. Then, together with students, practice finding the estimated difference by rounding the numbers in the given problems and solving the estimated problem. Next practice estimating the difference and then solving for the exact difference. Together, discuss that the estimated difference and the exact difference are quite close to each other. Then explain that you can also estimate the difference with multiple numbers (more than 2). Remind students that they can also choose to add the multiple subtrahends together to subtract them together from the minuend. Practice with exercises in context of miles, where you ask about how many miles are left to fly.
Check that students are able to estimate difference with numbers to 100,000 by asking the following questions:
- Why is it useful to round numbers and then subtract using rounded numbers?
- When do you round a number to the nearest thousand or ten thousands number?
- How do you round numbers like 12,289 and 49,305?