Rounding Decimals

The common rounding rule that “if the digit is over 5, go up” is in fact not always correct. Mindless application of this rule can lead to errors. For example to round 4.48 to the nearest whole number a common incorrect application of the rule is this: Round 4.48 to 4.5, round 4.5 to 5. Yet 4.48 is 4 to the nearest whole number. A reliable method about rounding is this: select the possible rounded number below and the possible rounded number above. See which of these the number to be rounded is closer to.

Using Materials

Problem: “Place 2.3429678335 on a number line between 2.34 and 2.35 as accurately  as possible.”

Write 2.3429678335 on the board and draw a number line with 10 divisions on the board as wide as the board. (The divisions need only be roughly equal, as measuring them will take too long. (All the better if the board is the width of the room!)

Discuss where 2.342 is. Draw 10 divisions between 2.342 and 2.343. Locate 2.3429.

Continue locating 2.3429678335 decimal place by decimal place until the interval is too small to continue.

 Examples: Worksheet (Material Master 8–14).

Using Number Properties

Examples: Round the following to a number of decimal places:

3.567 to 2 dps 12.5697 to 1 dp 109.780678 to 4 dps
0.002376 to 5 dps 10.09 to 1 dp 12.5067 to 3 dps
56.597 to 1 dp 0.0000371 to 5 dps 1.59709272 to 6 dps