Estimating and Money Problems
Approximate numerical solutions to problems involving rational numbers and percentages, including financial contexts, using appropriate estimation strategies. Use mathematical modelling to solve practical problems involving natural and rational numbers and percentages, including in financial contexts.
Read this first
Round to tidy numbers first. The estimate tells you if your exact answer is sensible.
An estimate of 57.
Words you need
- Approximate
- Close to, but not exact.
- Budget
- A plan for how to spend money.
- Model
- Turning a real problem into maths.
Worked example
e.g. Three items cost $18.95 each. Estimate, then find the exact total.
- Round 20.
- 3 lots of 60. That is the estimate.
- Now work it out exactly. 3 x 56.85.
- 60. The answer is reasonable.
Write the number sentence before you calculate. It keeps the steps clear.
Practice
The pictures fade as you go. By the last few you are on your own.
1Estimate 4 x $9.80.with a picture
AnswerAbout $40.
2You estimate 500. What happened?with a picture
AnswerThe answer is ten times too big. Check the working.
3Estimate 19% of $200.with a hint
Need a hint?
Round 19% to 20%.
AnswerAbout $40.
4You have 18.40 and $12.75. Estimate what is left.with a hint
Need a hint?
Round each amount.
AnswerAbout $19.
5Estimate 6 x $4.05.on your own
AnswerAbout $24.
6A $250 phone has 10% off. Estimate the saving.on your own
Answer$25.
7Why is estimating useful when shopping?on your own
AnswerYou can tell if you have enough money before you pay.
Think about it
1
When is an estimate good enough?
One good answerWhen you only need to know roughly, like checking you can afford something.
2
What does it mean if the exact answer is far from the estimate?
One good answerOne of them is wrong. Check both.
Common mistakes
Estimating after calculating.
Do it first, so it can catch a mistake.
Rounding everything up.
Round to the nearest, or the estimate drifts too high.