Add Decimals and Estimate the Answer

In this worksheet, you will be asked to round decimal numbers to the nearest whole number and then estimate the answer to an addition.

Recap of rounding:

What is 35 rounded to the nearest 10?

If the 'one' of the number is less than 5, the number needs to be rounded down.

If the 'one' of the number is 5 or above, the number needs to be rounded up.

So 35 would be rounded up to 40.

What is 834 rounded to the nearest 100?

834 would be rounded down to 800 because the tens digit is less than 50 so the number is rounded down.

What is 3.9 rounded to the nearest whole number?

Answer: 4

Look at the number after the decimal point, 9. Because it's more than 5, you round the 3 to the left of it, up to 4. 

What is 8.42 rounded to one decimal place?

Answer: 8.4

Look at the second digit after the decimal point, 2. Because it is less than 5, you leave the 4 to the left of it the same.

Example:

Estimate the answer to the addition by first rounding the decimal number.

10 + 1.42 = 10 + 1.4 = 11.4