Practise Rounding Decimals to Estimate Answers to Additions

In this activity, we will be rounding decimal numbers to the nearest whole number and then estimating the answer to an addition.

 

Recap of rounding:

What is 35 rounded to the nearest 10?

If the digit in the ones column is less than 5, the number needs to be rounded down.

If the digit in the ones column 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?

If the digit in the tens column is less than 5, the number needs to be rounded down.

If the digit in the tens column is 5 or above, the number needs to be rounded up.

834 would be rounded down to 800. 

 

What is 3.9 rounded to the nearest whole number?

If the digit in the tenths column (to the right of the decimal point) is less than 5, the number needs to be rounded down.

If the digit in the tenths column is 5 or above, the number needs to be rounded up.

3.9 wouold be rounded to 4

 

What is 8.42 rounded to one decimal place?

If the digit to the right of the tenths (the hundredths column) is less than 5, the number needs to be rounded down.

If the digit in the hundredths column is 5 or above, the number needs to be rounded up.

8.42 has a 2 in the hundredths column, so it rounds down to 8.4

 

 

Example:

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

 

10 + 1.42 rounded to one decimal place.

10 is already rounded.

1.42 rounds down to 1.4

 

10 + 1.42 = 10 + 1.4 = 11.4

 

Let's have a go at some questions now.