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Q&A #495 |
From: Gail
To: Teacher2Teacher Public Discussion
Date: 2000073118:32:05
Subject: Re: Rounding numbers
I want to second Mary's assertion that place value is an important part of rounding. I don't allow my fifth grade students to use the 0-4 and 5-9 "rule" when they round. Instead, they have to decide if the number they are working with is more, or less, than half way to the next consective ten / hundred / thousand or whatever place value we are rounding to. That means they need to be able to determine what the two consecutive multiples are "surrounding that amount (for example, if the amount is 1538, and we are rounding to the nearest hundred, then the two consecutive hundreds are 1500 and 1600. If we are rounding to the nearest thousand, then the two consecutive thousands are 1000 and 2000. Once they can identify the surrounding multiples, they need to be able to locate and name the midpoint on that line (for the above examples, 1550, or 1500), and then they need to be able to identify the location of the amount they are concerned with on a numberline (for the example amounts above, 1538 would be before the midpoint for 1550 and 1600, but after the midpoint for 1000 and 2000). This practice has paid off. Because we used the idea of "halfway" so often, now that we are multiplying decimal amounts, when we have a problem like .4 X .68, we know we can say that it is about "one half" times .68, or one half of .68. The understanding with regard to rounding has transferred and extended to a new topic. -Gail, for the Teacher2Teacher service
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