Rounding off Numbers

How do you round numbers like 32 and 38 to the nearest ten?

(1) First see what two tens a number is between. Both 32 and 38 are between 30 and 40.

(1) 首先，看看数字是介于哪两个十位数之间。32和38都介于30和40之间。

(2) Then see which ten a number is closer to. If the one's digit is less than 5, round to the lesser ten. If the one digit is 5 or greater, round to the greater ten.

(2) 然后再看哪一个十位数最接近你要四舍五入的数字。如果数字低于5，则四舍五入到较小的十位数。如果数字等于或是高于5，则四舍五入到较大的十位数。

From the rule, we know 32 rounds up to 30, and 38 rounds up to 40.

Estimation of Sums

To estimate a sum, first round each number to the same place value. Then add the rounded numbers.

Estimate: 53 + 48 = ?

Step 1: Round each number to the same place value.

53  →   50

53  →     50

+48  →+50

+  48  →+ 50

100

100

Estimate: 498 + 112 =?

Step 1: round each number to the same place value. Step 2: Add.

498  →     500

498  →     500

+  112  →+ 100

+112  →+ 100

600

600

【注】在商业上，概算（estimate)是一种重要的思考方式。人们通常运用概算的方式同他人进行商务交易。因此，在小学阶段要注意概算能力的提高。