Round each factor to a friendly number, then multiply the rounded numbers using a basic fact and the zeros. For 38 × 7, 38 rounds to 40, and 40 × 7 = 280. The estimate tells you roughly how big the exact answer should be, so you can check your working.
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
- factor
- a number that is multiplied
Worked examples
Estimate 38 × 7 by rounding 38 to the nearest ten.
- 1.38 rounds to 40.
- 2.4 × 7 = 28, then put on one zero: 280.
= 280
Estimate 48 × 31 by rounding both numbers to the nearest ten.
- 1.48 rounds to 50 and 31 rounds to 30.
- 2.5 × 3 = 15, then put on two zeros: 1500.
= 1500
Estimate 412 × 29 by rounding each number to its leading place.
- 1.412 rounds to 400 and 29 rounds to 30.
- 2.4 × 3 = 12, then put on three zeros: 12000.
= 12000
For parents
Method: Round the factors, then multiply
What to say: What do the numbers round to? What is the basic fact? How many zeros?
Common mistakes:
- Rounds by chopping off the last digits (always down), even when the number is closer to the next ten or hundred.
- Works out the exact product instead of estimating from rounded numbers.
- Loses a zero when multiplying the rounded numbers.
- Rounds only the first number and multiplies it by the second number unrounded.