Use Rounding to the Nearest Hundred to Check that Answers 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.

In this activity, we will be looking at how to round numbers to the nearest 100 in order to check that answers to addition and subtraction questions are reasonable.

Rules for rounding to the nearest 100:

When rounding to the nearest 100, we look at the tens digit.

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

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

Any digit in the ones column becomes a zero.

Let's look at some examples of rounding to the nearest 100.

Remember, we look at the tens column.

744 would be rounded down to 700 because there is a 4 in the tens column.

751 would be rounded up to 800 because there is a 5 in the tens column.

8,264 would be rounded up to 8,300  because there is a 6 in the tens column (even though this is a 4-digit number, we are still rounding to the nearest 100)

Now let's use rounding to check if the answer to a subtraction is reasonable:

1,232 - 673 = 559

Now, let's round the numbers:

1,232  rounds down to 1,200 (there is a 3 in the tens column)

673 rounds up to 700 (there is a 7 in the tens column)

1,200 - 700 = 500

So, 559 is a reasonable answer to the original question.

Now, it's your turn to have a go at some questions!

Don't forget - some will be addition and others subtraction!