Estimating a Sum
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
To estimate the sum we roundoff each number to the nearest tens and then add the roundedoff numbers.
Let us estimate 38 + 23.
38 is nearer to 40 than 30.
So, 38 is rounded up to 40.
The number 23 is nearer to 20 than 30.
So, 23 is rounded down to 20.
Solved Examples on Estimating the Sum:
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 to 10.
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
2. Round off the numbers 389 and 535 to the nearest 100s. Find
the estimated and the actual sum.
Solution:
Rounding off the numbers to nearest 100s
Hence, 400 + 500 = 900 is the estimated sum.
Questions and Answers on Estimating a Sum:
I. Estimate the sum. One has been done for you.
(i) 19 + 23
(ii) 27 + 44
(iii) 54 + 36
(iv) 41 + 38
(v) 52 + 17
(vi) 26 + 28
Answer:
I. (ii) 30 + 40 = 70
(iii) 50 + 40 = 90
(iv) 40 + 40 = 80
(v) 50 + 20 = 70
(vi) 30 + 30 = 60
II. Round off the given numbers to the nearest 100s. Find the estimated and the actual sum.

Estimated Sum

Actual Sum

(i) 582 and 157

__________

__________

(ii) 245 and 163

__________

__________

(iii)
812 and 111

__________

__________

(iv) 345 and 198

__________

__________

Answer:
II. (i) 800, 739
(ii) 400, 408
(iii) 900, 923
(iv) 500, 543
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