Question : Find the average of odd numbers from 9 to 831
Correct Answer 420
Solution & Explanation
Solution
Method (1) to find the average of the odd numbers from 9 to 831
Shortcut Trick to find the average of the given continuous odd numbers
The odd numbers from 9 to 831 are
9, 11, 13, . . . . 831
After observing the above list of the odd numbers from 9 to 831 we find that the difference between two consecutive terms are equal. This means the list of the odd numbers from 9 to 831 form an Arithmetic Series.
In the Arithmetic Series of the odd numbers from 9 to 831
The First Term (a) = 9
The Common Difference (d) = 2
And the last term (ℓ) = 831
The average of the numbers forming an Arithmetic Series
= The first term (a) + The last term (ℓ)/2
⇒ The average of numbers forming an Arithmetic Series = a + ℓ/2
Thus, the average of the odd numbers from 9 to 831
= 9 + 831/2
= 840/2 = 420
Thus, the average of the odd numbers from 9 to 831 = 420 Answer
Method (2) to find the average of the odd numbers from 9 to 831
Finding the average of given continuous odd numbers after finding their sum
The odd numbers from 9 to 831 are
9, 11, 13, . . . . 831
The odd numbers from 9 to 831 form an Arithmetic Series in which
The First Term (a) = 9
The Common Difference (d) = 2
And the last term (ℓ) = 831
The Average of the given numbers
= Sum of the given numbers/Total number of given numbers
Thus, to find the average of the given numbers, first, we need to find their sum and the total number of given numbers
Finding the number of terms
For an Arithmetic Series, the nth term
an = a + (n – 1) d
Where
a = First term
d = Common difference
n = number of terms
an = nth term
Thus, for the given series of the odd numbers from 9 to 831
831 = 9 + (n – 1) × 2
⇒ 831 = 9 + 2 n – 2
⇒ 831 = 9 – 2 + 2 n
⇒ 831 = 7 + 2 n
After transposing 7 to LHS
⇒ 831 – 7 = 2 n
⇒ 824 = 2 n
After rearranging the above expression
⇒ 2 n = 824
After transposing 2 to RHS
⇒ n = 824/2
⇒ n = 412
Thus, the number of terms of odd numbers from 9 to 831 = 412
This means 831 is the 412th term.
Finding the sum of the given odd numbers from 9 to 831
The sum of all terms (S) in an Arithmetic Series
= n/2 (a + ℓ)
Where, n = number of terms
a = First term
And, ℓ = Last term
Thus, the sum of all terms (S) of the given odd numbers from 9 to 831
= 412/2 (9 + 831)
= 412/2 × 840
= 412 × 840/2
= 346080/2 = 173040
Thus, the sum of all terms of the given odd numbers from 9 to 831 = 173040
And, the total number of terms = 412
Since, the average of the given numbers
= Sum of the given numbers/Total number of given numbers
Thus, the average of the given odd numbers from 9 to 831
= 173040/412 = 420
Thus, the average of the given odd numbers from 9 to 831 = 420 Answer
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