Estimating with decimals, fractions and percentages
An estimate uses friendly numbers so you can calculate in your head. Round each number to the nearest whole number, ten or dollar, as the question says: 5 or more rounds up. Then calculate with the rounded numbers. For percentages, find 10% by dividing by 10, 50% by halving and 25% by halving twice. For a fraction, divide by the bottom number and multiply by the top. Estimates are close to the exact answer, not equal to it.
Explore this idea with maths tools
Maths you can move
Build 8 + 5
Tap two gold counters to fill the ten-frame.
8 in the frame + 5 waiting. The total stays the same.
Words to know
- estimate
- a close answer found with friendly numbers
- round
- change a number to a nearby friendly number
- percentage
- a part out of 100, such as 10%
Worked examples
Estimate 4.9 × 6.1 by rounding each number to the nearest whole number.
- 1.4.9 rounds to 5 and 6.1 rounds to 6.
- 2.5 × 6 = 30.
= 30
Estimate 10% of $39.95. Round the price to the nearest $10 first.
- 1.$39.95 is closest to $40.
- 2.10% of 40 means divide by 10: 4.
= 4
A jacket costs $79.95 and is 20% off. Estimate the sale price. Round the price to the nearest $10 first.
- 1.$79.95 is closest to $80.
- 2.10% of 80 = 8, so 20% = 16.
- 3.80 - 16 = 64.
= 64
For parents
Method: Round first, then calculate in your head. Use 10% as the building block.
What to say: Which friendly number is it closest to? Does the answer look reasonable for the price in the shop?
Common mistakes:
- Chops off the decimal part instead of rounding to the nearest whole number or friendly number.
- Rounds every number up, even when the nearest friendly number is lower.
- Rounds correctly but then uses the wrong operation or finds the wrong part, for example adding when the problem multiplies, or finding what is left instead of the part asked for.
- Stops one step early: leaves out a price, forgets the numerator, gives the discount or GST instead of the final price.