In this post, we are going to** learn how to round and use estimation in subtraction problems**. Mastering this skill is helpful in verifying that an exact answer is of the correct order of magnitude and also in determining whether an answer is reasonable or not.

### Round and Estimating Subtraction Problems in Tens

- Round the numbers that numbers that will be subtracted to the nearest ten: if the number ends in 0, 1, 2, 3 or 4, we round down. If the number ends in 5, 6, 7, 8 or 9, we round up
- Subtract the rounded numbers.
- Even out the amount that was rounded. The rounded quantity is the number that we’ve lessened or added to the number in order to round.

- If both numbers were rounded up or down, we don’t have to do anything more
- If the first number was rounded down and the second was rounded up, and the total of the rounded amounts is 5 or greater, we have to add 10 to the estimation.
- If the first number was rounded up and the second down, and the total of the rounded amounts is 5 or greater, we have to subtract 10 from the estimation.

**Example: 65 – 32**

- Round the two numbers:

65 –> 70

32 –> 30

- Subtract the rounded numbers:

70 – 30 = 40

- Even out the rounded numbers:

**65 –> 70 32 –> 30**

The first number was rounded up and the second was rounded down, so we need to even out the rounded quantity:

65 –> 70 32 –> 30

5 2

Then add the two numbers:

5 + 2 = 7

The sum of the rounded amounts turns out to be greater than 5, so we need to subtract 10 from the estimation:

40 – 10 = 30

**The correct estimation then, is 30.**

### Round and Estimating subtraction problems in hundreds

- Round the numbers that numbers that will be subtracted to the nearest hundred.
- Subtract the rounded numbers.
- Even out the amount that was rounded.

- If both numbers were rounded up or down, we don’t have to do anything more
- If the first number was rounded down and the second up, and the total of the rounded amounts is 50 or greater, we have to add 100 to the estimation.
- If the first number was rounded up and the second down, and the total of the rounded amounts is 5 or greater, we have to subtract 100 from the estimation.

**Example: 658 – 171**

- Round the two numbers:

658 –> 700

171 –> 200

- Subtract the rounded numbers:

700 – 200 = 500

- Even out the rounded quantity:

658 –> 700 171 –> 200

Both numbers were rounded up so you don’t need to change anything; **the correct estimation is 500. **

I hope you liked this post! If you want to keep learning, you can try Smartick’s free trial.

Learn More:

- Learn How to Estimate Sums with Some Examples
- Learn How to Estimate a Sum with Examples
- Estimations: Rounding Decimals to The Nearest Whole Number
- Learn to Subtract with Remainders
- Strategy for Mental Calculation: Add and Subtract Numbers

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