Checking Answers by Rounding
Use rounding to check answers to calculations and determine appropriate levels of accuracy in context
What a learner can do afterwards
- Check 47,832 + 23,156 ≈ 48,000 + 23,000 = 71,000 to verify the exact answer 70,988
- Decide whether to round to the nearest 100 or 1,000 for a given estimation context
- Identify that a calculated answer of 3,421 cannot be correct because the estimate gives approximately 50,000
1 · Read
After you calculate an answer, you can check it without redoing the whole problem. Round the numbers first, then do a quick estimate. If your exact answer is close to the estimate, you probably got it right.
How much you round depends on the numbers and how careful you need to be. For very large numbers, round to the nearest 1,000 for a fast estimate. For smaller numbers, or when you want a tighter check, round to the nearest 100 instead.
Someone adds two numbers and gets 3,421. Rounding the numbers first gives an estimate of about 50,000. Since 3,421 is nowhere close to 50,000, the answer must be wrong. Maybe a digit got left out.
If your exact answer and your estimate are far apart, do not trust the exact answer. Go back and calculate again.
Round the numbers, estimate the answer, and compare: close means you're probably right, far apart means check your work again.
More to read
1.1 Introduction to Whole Numbers
Whole numbers are the counting numbers 0, 1, 2, 3 and so on, with no fractions allowed. Each place in a number is worth ten times the place to its right. That is why the 2 in 243 means 200, not just 2.
Place value lets you read, compare and round big numbers. Once you can name the value of each digit, thousands and ten thousands stop looking scary.
Add and subtract within 1,000,000
Big additions and subtractions use the same column method as small ones. Stack digits by place, work right to left, and carry or exchange as needed. Size changes nothing.
Keep columns tidy past the thousands. One wobbly digit can cost you ten thousand.
Key ideas: Estimate products and quotients by using powers of 10 and their multiples
Estimates use friendly round numbers. Swap 48,600 for 50,000 and the multiplication turns easy. The answer will be close, not exact.
Powers of ten are the friendliest numbers of all. Multiplying by 10, 100 or 1,000 just adds zeros.
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