Rounding

Definitions

Benchmark Numbers: Two numbers that help estimate or round

Midpoint: The number in the middle of the benchmark numbers

Rounding: An estimate of a number to the closest benchmark number

Round Up: Round up to the greatest benchmark number when on or past the midpoint

Round Down: Round down to the least benchmark number when before the midpoint

Nearest 10: Rounding to a count by 10 number

Nearest 100: Rounding to a count by 100 number

Benchmark Numbers & Midpoint

Benchmark Numbers

These are two numbers on a number line that are typically count by 10's or 100's numbers when rounding. Benchmark numbers are the specific numbers that the given number falls between. If you are rounding 23 to the nearest ten, the benchmark numbers are 20 and 30, where as if you are rounding 23 to the nearest 100, the benchmark numbers are 0 and 100.

Midpoint

The midpoint is the number directly in the center, or middle, of the two benchmark numbers. This point helps you know whether to round up or round down. If the given number is equal to or greater than the midpoint, you round up. If the given number in less than the midpoint, you round down.

Ex. Round 23 to the nearest 10

(See vertical number line to the right.) 23 falls between 20 and 30, and does not pass the midpoint of 25. Therefore, 23 rounds down to 20 because it is closer to that benchmark number.

Vertical Number Line

Mountain Model

This model is a great visual for students who are having a hard time deciding when to round up and when to round down. The Midpoint is at the TOP of the mountain with the Benchmark Numbers at the two bases. If the number doesn't reach the top, you round down to the lesser benchmark number. If the given number is at the top or on the other side of the mountain, you round up to the greater benchmark number at the other base.

See example with rounding 23 to the nearest ten. (left)

Page updated

Report abuse