1
Estimating totals from a chart
Discover Mathematics P3: Term 1, Lessons 51–60
GoalRound numbers to the nearest hundred and use them to estimate totals.
New words
estimate · an answer that is close to the exact answerround · to change a number to a nearby friendly numbernearest hundred · the closest multiple of 100
Explanation
An estimate is an answer that is close to the exact answer. To estimate a sum, round each number to the nearest hundred and then add. To round to the nearest hundred, look at the tens digit. If it is 5 or more, round up. If it is less than 5, round down. So 412 rounds to 400, 289 rounds to 300, 603 rounds to 600 and 198 rounds to 200. To estimate 412 + 289, add 400 + 300 = 700. The exact sum is 701, so the estimate is very close. An estimate helps us check that an exact answer is sensible.
Worked examples
| Day | Boxes | Rounded |
|---|---|---|
| Sat | 412 | 400 |
| Sun | 289 | 300 |
| Mon | 603 | 600 |
| Tue | 198 | 200 |
Sunday and Monday: 289 rounds to 300 and 603 rounds to 600, so the estimate is 300 + 600 = 900. The exact sum is 289 + 603 = 892.
Saturday and Monday: 412 rounds to 400 and 603 rounds to 600, so the estimate is 400 + 600 = 1,000. The exact sum is 412 + 603 = 1,015. These two days sold about 1,000 boxes.
Class activity
Estimate first. Write down the prices or weights of 4 things from a shop leaflet or from home, such as 3-digit amounts. Pick any two, round each to the nearest hundred and estimate the total. Then add them exactly and see how close your estimate was.
Practice
1
Round each number to the nearest hundred: 345, 678, 250 and 951.
345 rounds to 300, 678 rounds to 700, 250 rounds to 300 and 951 rounds to 1,000.
2
Estimate 287 + 412.
287 is about 300 and 412 is about 400, so the estimate is 700. The exact sum is 699.
3
Estimate 523 + 189.
523 is about 500 and 189 is about 200, so the estimate is 700. The exact sum is 712.
4
Hana picks 468 guavas and 231 oranges. About how many fruits is that?
468 is about 500 and 231 is about 200, so about 500 + 200 = 700 fruits. The exact number is 699.
5
Youssef says 312 + 289 is about 1,000. Is this a sensible estimate?
No. 312 is about 300 and 289 is about 300, so the estimate is 600. The exact sum is 601.