We learn the basics of estimating a sum. Here we learn an easy way to estimate a sum of two numbers by rounding them up.

In the case of two-digit numbers, we can only round the number to the nearest ten, that is, only estimate one place.

For example, we estimate the following sums:

(i) 47 + 32

We need to round the number up to the nearest 10.

47 → 50

32 → 30

50 + 30 = 80

(ii) 25 + 34


We need to round the number up to the nearest 10.

25 → 30

34 → 30

30 + 30 = 60

(iii) 75 + 13

We need to round the number up to the nearest 10.

75 → 80

13 → 10

80 + 10 = 90

1. Estimate the sums of 72 and 48

Solution:

We round the number to the nearest 10.

72 → 70

48 → 50

So 72 is the closest to 70 and 48 is the closest to 50

70 + 50 = 120

So the estimated sum = 120

2. Estimate the sums to the nearest ten and also find the actual sum of 87 and 79.

Solution:

We round the number to the nearest 10.

87 → 90

79 → 80

So 87 is closest to 90 and 79 is closest to 80

90 + 80 = 170

So the estimated total = 170

The actual sum of 87 and 79

87 + 79 = 166

So the actual total is 166

Note:To estimate to the next 10, we see the digit / number in its own place. It will be converted to 0 or 10 accordingly. If the digit / number < 10/2 ist, wird sie in Null umgewandelt, und wenn sie > Is 10/2, it is converted to 10.

In the case of three-digit numbers, we can only round the number to the nearest ten, that is, only estimate one place.

For example, we estimate the following sums:

1. Estimate the sums of 586 and 120

Solution:

We’ll round the number to the nearest hundred.

586 → 600

120 → 100

So, 586 is a hundred closest is 600 and 120 is a hundred closest to 100

600 + 100 = 700

So the estimated sum to the next hundred= 700

We round the number to the nearest ten.

586 → 590

120 → 120

So, 586 is ten nearest is 590 and 120 is nearest hundred is 120

590 + 120 = 710

So the estimated sum over the next ten= 710

2. Round up the numbers 389 and 535 to the nearest 100. Find the estimated and actual total.

Solution:

Round off the numbers to the nearest 100

Estimate a sum

So 400 + 500 = 900 is the estimated total.

Questions and answers about estimating a sum:

I. Round the numbers shown to the nearest 100s. Find the estimated and actual total.

Estimated sum

Actual amount

(i) 582 and 157

__________

__________

(ii) 245 and 163

__________

__________

(iii) 812 and 111

__________

__________

(iv) 345 and 198

__________

__________

Reply:

(i) 800, 739

(ii) 400, 408

(iii) 900, 923

(iv) 500, 543

3rd grade math class

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