Lesson Page Uncertainty

Rounding is one way to estimate sums and difference.

Round 53, 55, and 58 to the nearest ten. Estimate the sum.

 50 51 52 53 54 55 56 57 58 59 60

50

53 is between 50 and 60. It is closer to 50 Round down to 50.

55 is halfway between 50 and 60. Round up to 60.

58 is between 50 and 60. It is closer to 60. Round up to 60.

60

53      50

55      60
+58  + 60

?             170

 To round numbers: Find the place you are rounding to. Look at the digit to its right. If the digit to the right of the place one is rounding to is less than 5, round down; if it is 5 or greater, round up. Then add or subtract.

Example: Round to the nearest ten.

 Estimate the sum. Estimate the difference. 36  +44      ? 40 +40 80 52 -25  ? 50 -30  20

Example: Round to the nearest hundred.

 Estimate the sum. Estimate the difference. 256  +444      ? 300 +400  700 536  -154      ? 500 -200  300

 Remember to round both numbers to the same place value when estimating a sum or a difference. When a number is halfway between two numbers, round to the greater number.

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