Idea 12of 24Builds onTimes 0.1 makes a number smaller
Estimate a decimal sum
How can you add decimals in your head, roughly?
Today you'll discover
- how to round to a whole number
- how to add the rounded numbers
- how an estimate checks an answer
Think first3.82 + 4.15, roughly
About how much is 3.82 + 4.15?
Decide first, then press Next. Look at the tenths digit to round.
A number line from 3 to 5 with dots at 3.82 and 4.15. Curved arrows take each dot to 4, boxed under the line. Then the estimate 4 + 4 = about 8, and the exact sum 3.82 + 4.15 = 7.97, close to 8. The last step shows 2.67 and 5.48 on a line from 2 to 6.
See the whole ideathe big idea and the key word, in one place
The big idea
Round each decimal to the nearest whole number, then add: a quick estimate.
Easy to get wrongRound, don't chop. 3.82 rounds to 4, not 3: look at the tenths digit, 8.
A slip, caughtAn answer of 79.7 for 3.82 + 4.15 cannot be right: the estimate says about 8.
Key word
- estimate
- a close answer found quickly, often by rounding first
Ready to use it?
- Estimate a sum of decimal numbers.
- Use an estimate to check an exact answer.