Estimation to check answers to calculations
Use estimation to check answers to calculations and determine, in the context of a problem, an appropriate degree of accuracy
What a learner can do afterwards
- Estimate 487 × 23 by rounding to 500 × 20 and use this to check a calculated answer
- Determine whether an exact answer or an estimate is more appropriate in a given context
- Spot an unreasonable answer by comparing with a quick mental estimate
The lesson
You already know how to round numbers to make a quick estimate. Now you'll use that same idea to check your calculations, especially when you multiply or divide bigger numbers.
To estimate 487 x 23, round each number first: 487 rounds to 500, and 23 rounds to 20. So 500 x 20 = 10,000 is your estimate. The exact answer is 11,201. Since 11,201 is close to 10,000, the exact answer looks about right.
Sometimes you need the exact answer, and sometimes an estimate is enough. If you're working out exact change or exact money, you need the precise number. If you just want a rough idea, like roughly how much paint to buy, an estimate does the job.
If your worked-out answer is way off from your estimate, like ten times too big or too small, check your work. A missing digit or a slipped decimal point is often the cause.
Round first to make an estimate, then check your exact answer against it, and decide whether the situation needs exact or roughly right.
Watch it
Where it sits
This opens up
8 questions wait behind this lesson, each with its answer explained. Every answer feeds the sky: stars light as they are learned, and dim when it is time to come back.