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Students are told that rounding makes numbers easier. But rounding is a very abstract notion that demands a strong foundation in place value. Typically, teaching students rules like 5 and above round up, 4 and below round down, tend to confuse more students than they help unless it is explained conceptually.
Students are each given the following hundreds chart. The solid line running through the chart shows students how to decide which ten to round to. Students are told that they will be representing numbers on the hundreds chart using base ten blocks.
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The teacher demonstrates representing the number 33 using tens blocks and ones blocks.
When rounding to the nearest ten, student are ridding the number of its ones so that it is easier to compute. After representing the number with base ten blocks, students are directed to write the number, and the two tens the number is between:
30 33 40
Students are told that if the ones fall to the left of the line, the ones are removed and the number is rounded to the lesser ten. In this case, the three ones fall to the left of the solid line, so they are removed, leaving a value of 30.
The bottom example shows that the three ones were taken away, leaving 3 tens or 30. This is a rounded number. |
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Any number with ones right of the solid line round to the next highest ten. It is logical to the students that adding two more ones to 68 is the easiest way to get to the next ten, 70, as opposed to getting rid of 8 ones. |
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Students write:
60 68 70
When the students add two more ones, the number has been rounded to 70. The ones are replaced with one ten, leaving 7 tens total. |
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Students are presented with a problem like the following to put rounding and estimating in context:
At the school store, a pencil costs 33 cents and an eraser costs 68 cents. About how much do they cost total?
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=100 |
| Students can also use the same method to round three digit numbers to the nearest ten. Often, students struggle with rounding three digit numbers, but when they see that they can use the same procedure used for two digit numbers, the procedure makes sense. Students can also round to the nearest hundred, getting rid of all of the tens and ones by either rounding to 200 or to 300. |
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Above, the students completed the activity using the Kidspiration program on the computer. Students in Ms. Frey's class completed the activity using base ten blocks.
Indicators:
6.3.5.5 estimate sums and
differences of whole numbers less
than 1,000.
6.2.7.2 estimate quantities to 100.
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