Use Rounding to Check that Answers to Addition and Subtraction Problems are Reasonable

When adding and subtracting larger numbers, it can be useful to be able to check to see if our answers are reasonable. 

Rounding the numbers we are adding/subtracting is a useful way of doing this.

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In this activity, we will be rounding numbers in order to check that answers to addition and subtraction problems are reasonable.

Rules for rounding

When rounding, we look at the  digit to the right  of the number we are rounding to. 

If we are rounding to a thousand we look at the digit in the hundreds column; when rounding to the nearest hundred we look at the tens column etc.

If this digit is less than 5, the number is rounded down.

If this digit is 5 or more, the number is rounded up.

Any digit in the columns to the right of this digit becomes a zero.

Let's look at some examples:

744 rounded to the nearest 100 would  be rounded down to 700 

6,432  rounded to the nearest 1,000 would be rounded down to 6,000

18,264 rounded to the nearest 1,000 would be rounded down to 18,000  

18,264 rounded to the nearest 10,000 would be rounded up to 19,000

Now let's use rounding to check that the answer to a problem is reasonable.

The population of North Town is 453,210

The population of South Town is 329,991

Round the numbers to the nearest 10,000 to check  if the actual answer is reasonable.

First, calculate the answer:  453,210 + 329,991 = 783,201

Now, let's round the numbers to the nearest 10,000 and then add them to see if the answer is reasonable.

450,000 + 330,000 = 880,000

Looking at the estimate, we can assume that 783, 201  is probably the correct answer as it is quite close to the estimate. 

If the answer was very different we could assume that we had made an error!

Now, it's your turn to have a go at some questions like this.  Good luck!