Understand the idea.
Get a sensible estimate
An estimate is a quick calculation using nearby, easier numbers. Round each price to the nearest ten dollars before adding. A 28-dollar bag becomes about 30 dollars and a 13-dollar book becomes about 10 dollars, giving about 40 dollars altogether.
Rounding changes the amounts, so the estimate is not the shop price. Say which rounding you used. Nearest-ten estimates suit larger whole-dollar prices; rounding to the nearest dollar may be more useful for small prices.
Check decisions near the limit
Use an estimate to decide whether an exact answer looks sensible. The exact calculation dollars is close to 40 dollars. An answer of 141 dollars is much too far away and needs checking.
Near a budget, calculate exactly before deciding. Having 40 dollars does not cover a 41-dollar purchase, even though the estimate was 40 dollars. Rounding can raise one price and lower another, so an estimate may be above or below the real total.
See the method in action.
Estimate and compare
A game costs 47 dollars and a ball costs 22 dollars. Estimate the total to the nearest ten, then calculate exactly.
Worked solution
- 1
Round each price to the nearest ten.
47 is nearer 50 than 40; 22 is nearer 20 than 30.
- 2
Add the rounded prices.
This gives an estimate using friendly tens.
- 3
Add the original prices exactly.
Splitting 22 into 20 and 2 keeps the original price unchanged.
- 4
Compare the two totals.
The exact total is only 1 dollar below the estimate, so the answers agree sensibly.
Estimate 70 dollars; exact total 69 dollars.
Check a close budget
Mina has 50 dollars. She estimates that items costing 26 and 25 dollars will fit her budget. Check exactly.
Worked solution
- 1
Find the exact cost using the original prices.
The prices are 26 dollars and 25 dollars, so their exact sum determines what Mina must pay.
- 2
Compare the exact cost with the budget.
The items cost more than the 50 dollars available, so Mina cannot buy both with this budget.
- 3
Find how much more is needed.
The exact cost exceeds her budget by 1 dollar; the estimate that the items would fit was not enough for this decision.
She does not have enough; she is 1 dollar short.
Try it yourself.
Check your understanding
Estimate 38 + 21 dollars by rounding each price to the nearest ten.
Show answer & reasoning
60 dollars
Round each price first: and . Then dollars. This is an estimate, not the exact 59-dollar total.
Keep practising
6 worksheet questions · 11 marks
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 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 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.
dollars.
70 dollars - Calculate the exact total of 29 and 42 dollars. Compare it with your estimate.2 marks
.
.
71 dollars; 1 dollar above the 70-dollar estimate. - A learner writes dollars. Use an estimate to explain why this cannot be right.2 marks
is a useful estimate.
, 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 settle whether you can pay? Check exactly.2 marks
The estimate is 40 but rounding hid 1 dollar.
.
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; .
.
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 ↗