The idea, step by step
Get a sensible estimate
An estimate is a nearby amount that is easier to calculate. A bag costing 28 dollars is about 30 dollars. A book costing 13 dollars is about 10 dollars. Their estimated total is 40 dollars.
Say what you rounded to. Rounding prices to the nearest ten is useful for a quick whole-dollar estimate. For small prices, the nearest dollar can be more useful. An estimate is not the amount the shop must charge.
Check decisions near the limit
The exact total of 28 and 13 dollars is 41 dollars. If you have 40 dollars, the estimate of 40 does not prove you have enough. Work out the exact cost before paying or making a close budget decision.
Use an estimate to catch a large mistake. If you calculate 28 + 13 as 141, the estimate near 40 tells you to check. Estimates can be above or below the exact answer; they are not always money left over.
A common trap
My estimate equals my budget, so the purchase must be affordable.
Instead: A rounded estimate can be too low. Compare the exact total with the budget when the amounts are close.
See the thinking
Estimate and compare
A game costs 47 dollars and a ball costs 22 dollars. Estimate the total to the nearest ten, then calculate exactly.
- 47 rounds to 50 and 22 rounds to 20.
- Estimated total: 50 + 20 = 70 dollars.
- Exact total: 47 + 22 = 69 dollars, close to the estimate.
Check a close budget
Mina has 50 dollars. She estimates that items costing 26 and 25 dollars will fit her budget. Check exactly.
- Rounding both prices to the nearest ten gives 30 + 30 = 60 dollars, so that estimate warns the cost may be too high.
- Add the exact prices: 26 + 25 = 51 dollars.
- 51 is 1 dollar more than the budget.
Check your understanding
Estimate 38 + 21 dollars by rounding each price to the nearest ten.
Show answer
60 dollars
38 is about 40 and 21 is about 20; 40 + 20 = 60.
Your turn
6 questions · 11 marks · Work at your own pace.
Read the questions online
- Round a price of 63 dollars to the nearest ten dollars.1 mark
- Estimate the total of 29 and 42 dollars, rounding each to the nearest ten.2 marks
- Calculate the exact total of 29 and 42 dollars. Compare it with your estimate.2 marks
- A learner writes 54 + 18 = 172 dollars. Use an estimate to explain why this cannot be right.2 marks
- You have 40 dollars. Two items cost 19 and 22 dollars. Does the estimate 20 + 20 settle whether you can pay? Check exactly.2 marks
- A bag costs 68 dollars. Estimate the change from 100 dollars to the nearest ten, then find the exact change.2 marks
Read the worked solutions
Equivalent correct methods earn the corresponding marks. Include units where asked.
- Round a price of 63 dollars to the nearest ten dollars.1 mark
63 is closer to 60 than 70.
60 dollars - Estimate the total of 29 and 42 dollars, rounding each to the nearest ten.2 marks
29 rounds to 30; 42 rounds to 40.
30 + 40 = 70 dollars.
70 dollars - Calculate the exact total of 29 and 42 dollars. Compare it with your estimate.2 marks
29 + 42 = 71.
71 − 70 = 1.
71 dollars; 1 dollar above the 70-dollar estimate. - A learner writes 54 + 18 = 172 dollars. Use an estimate to explain why this cannot be right.2 marks
50 + 20 = 70 is a useful estimate.
54 + 18 = 72, not 172.
The total should be near 70 dollars; the exact answer is 72 dollars. - You have 40 dollars. Two items cost 19 and 22 dollars. Does the estimate 20 + 20 settle whether you can pay? Check exactly.2 marks
The estimate is 40 but rounding hid 1 dollar.
19 + 22 = 41.
No; the exact total is 41 dollars, so you are 1 dollar short. - A bag costs 68 dollars. Estimate the change from 100 dollars to the nearest ten, then find the exact change.2 marks
68 rounds to 70; 100 − 70 = 30.
100 − 68 = 32.
Estimated change 30 dollars; exact change 32 dollars.
Curriculum reference & lesson scope
Australian Curriculum v9 and SA Curriculum Mathematics 2026
Estimating transactions; multi-step models; responsible purchases; class enterprise and community effects.
Official curriculum source ↗