We will learn the basic knowledge for estimating a sum. Here we will learn an easy way to estimate a sum of two numbers by rounding.
In case of two digit numbers we can only round the number to the nearest tens place i.e. only one place estimate.
For example, let us estimate the following sums:
(i) 47 + 32
We need to round the number to the nearest 10.
47 → 50
32 → 30
50 + 30 = 80
(ii) 25 + 34
We need to round the number to the
nearest 10.
25 → 30
34 → 30
30 + 30 = 60
(iii) 75 + 13
We need to round the number 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 nearest to 70 and 48 is nearest to 50
70 + 50 = 120
Thus 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 nearest to 90 and 79 is nearest to 80
90 + 80 = 170
Thus the estimated sum = 170
The actual sum of 87 and 79
87 + 79 = 166
Thus the actual sum = 166
Note: To estimate to the nearest 10, we see the digit/number at one’s place. It is converted to 0 or 10 as proper. If the digit/number is < 10/2, it is converted to zero and if it is > 10/2, it is converted to10.
In case of three digit numbers we can only round the number to the nearest tens place i.e. only one place estimate.
For example, let us estimate the following sums:
1. Estimate the sums of 586 and 120
Solution:
We round the number to the nearest hundred.
586 → 600
120 → 100
So, 586 is nearest to hundred is 600 and 120 is nearest to hundred 100
600 + 100 = 700
Thus the estimated sum to nearest hundred = 700
We round the number to the nearest tens.
586 → 590
120 → 120
So, 586 is nearest to ten is 590 and 120 is nearest to hundred 120
590 + 120 = 710
Thus the estimated sum to nearest ten = 710
From Estimating a Sum to HOE PAGE
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