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 twodigit 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
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 threedigit 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
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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