Mark Zegarelli - Basic Math & Pre-Algebra All-in-One For Dummies (+ Chapter Quizzes Online)

Occasionally, a small change to the ones and tens digits affects the other digits. (This situation is a lot like when the odometer in your car rolls a bunch of 9s over to 0s.) For example:

To round numbers to the nearest hundred, thousand, or beyond, focus only on two digits: the digit in the place you’re rounding to and the digit to its immediate right. Change all other digits to the right of these two digits to 0s. For example, suppose you want to round 642 to the nearest hundred. Focus on the hundreds digit (6) and the digit to its immediate right (4):

I’ve underlined these two digits. Now just round these two digits as if you were rounding to the nearest ten, and change the digit to the right of them to a 0:

Here are a few more examples of rounding numbers to the nearest hundred:

When rounding numbers to the nearest thousand, underline the thousands digit and the digit to its immediate right. Round the number by focusing only on the two underlined digits and, when you’re done, change all digits to the right of these to 0s:

Even when rounding to the nearest million, the same rules apply:

1 Round the following numbers to the nearest ten:

(a)83

(b)217

(c)1,885

(d)6,496

2 Round the following numbers to the nearest hundred:

(a)347

(b)2,251

(c)7,950

(d)39,974

3 Round the following numbers to the nearest thousand:

(a)6,543

(b)9,287

(c)22,501

(d)799,643

4 Round the following numbers to the nearest million:

(a)5,454,545

(b)10,730,421

(c)98,765,432

(d)5,555,555,555

When you know how to round numbers, you can use this skill in estimating values. Estimating saves you time by allowing you to avoid complicated computations and still get an approximate answer to a problem.

When you get an approximate answer, you don’t use an equals sign; instead, you use this wavy symbol, which means is approximately equal to: ≈.

Suppose you want to add these number: . This computation is tedious, and you may make a mistake. But you can make the addition easier by first rounding all the numbers to the nearest hundred and then adding:

The approximate answer is 3,800. This answer isn’t far off from the exact answer, which is 3,792.

5 Estimate by rounding all values to the nearest ten before adding.

6 Estimate by rounding all values to the nearest hundred before adding.

7 Estimate by rounding all values to the nearest hundred before adding and subtracting.

8 Estimate by rounding all values to the nearest thousand before adding and subtracting.

1

(a) 80.

(b) 220.

(c) 1,890.

(d) 6,500.

2

(a) 300.

(b) 2,300.

(c) 8,000.