Whole Number Arithmetic Online Documentation 
Use Estimating Skills 
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Learners can estimate the answer to a problem when they do not need an exact number answer. They need to know which situations require exact answers. Estimation arrives at a sometimes accurate, but usually only approximate, answer to computation problems. Learners can learn to estimate an answer to be more, or to be less, than the exact total. You can present examples of this in many situations. For example, when shopping in a store, pose problems such as: You know that pencils cost 24 cents and you have 60 cents. Do you have enough to buy one; two; three? If you estimate by rounding 24 to 20, you might estimate that you have enough money to buy three pencils (3 * 20 = 60). If you change 24 to 30, you would determine that you only have enough to buy two pencils. In this case it is all right to estimate, but you have to be careful not to make the error of estimating too small a number, which might leave you not able to buy what you expected. So in this case, it might be good to use the rule when shopping, if you only have limited amount, to always estimate high. Then add what you have for a total, and make sure it is less than what you have to spend, and you will have enough. Learners can also estimate to check answers to problems. For example if the problem is :
You know that your answer has to be more than : 200 x 10,000, or 2,000,000 If you answered with a smaller number, there is an error. There are many ways to estimate numbers. Each learner may develop individualized methods for estimating. These methods include:
using frontend numbers clustering numbers Rounding If you do not need specific numbers, try rounding. The usual rule for rounding is that if digit is 5 or more, round up. If the digit is less than 5, round down (98 becomes 100, 12 becomes 10, etc.). Follow these steps to round a number:
Front End Estimation You usually work addition, subtraction, and multiplication problems from right to left, starting with numbers in the ones place (for more definition, see place value in the Glossary). However, when estimating, it is sometimes useful to start with numbers at the front end, or in the leftmost position.
For example, to estimate the sum of these numbers:
507 654 849
54 + 49 is approximately 100 Add the 200 to 2300. The answer is approximately 2500. By adding just the leftmost number, you at least know a number that your total must be larger than. For example, to add the numbers: 856 + 234 + 456, add 8+2+4. You know that the final sum will be more than 1400. You can also estimate differences in subtraction using front end estimation. For example in this problem: 9436  8654 First, estimate 9,000  8,000 = 1,000 (9  8 = 1) Then, compare the rest. Since 436 is less than 654, the first estimate (1,000) is about 200 too much. So, adjust the first estimate by subtracting 200 from 1000, and the final estimate is 800. Clustering
Look at numbers to find a number that they cluster around, or are close to. For example, to use clustering to add these numbers:
603 622 586 You can estimate by figuring 4 X 600, which makes the estimate 2400. 