Question:
Find the average of odd numbers from 9 to 1287
Correct Answer
648
Solution And Explanation
Solution
Method (1) to find the average of the odd numbers from 9 to 1287
Shortcut Trick to find the average of the given continuous odd numbers
The odd numbers from 9 to 1287 are
9, 11, 13, . . . . 1287
After observing the above list of the odd numbers from 9 to 1287 we find that the difference between two consecutive terms are equal. This means the list of the odd numbers from 9 to 1287 form an Arithmetic Series.
In the Arithmetic Series of the odd numbers from 9 to 1287
The First Term (a) = 9
The Common Difference (d) = 2
And the last term (ℓ) = 1287
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 1287
= 9 + 1287/2
= 1296/2 = 648
Thus, the average of the odd numbers from 9 to 1287 = 648 Answer
Method (2) to find the average of the odd numbers from 9 to 1287
Finding the average of given continuous odd numbers after finding their sum
The odd numbers from 9 to 1287 are
9, 11, 13, . . . . 1287
The odd numbers from 9 to 1287 form an Arithmetic Series in which
The First Term (a) = 9
The Common Difference (d) = 2
And the last term (ℓ) = 1287
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 1287
1287 = 9 + (n – 1) × 2
⇒ 1287 = 9 + 2 n – 2
⇒ 1287 = 9 – 2 + 2 n
⇒ 1287 = 7 + 2 n
After transposing 7 to LHS
⇒ 1287 – 7 = 2 n
⇒ 1280 = 2 n
After rearranging the above expression
⇒ 2 n = 1280
After transposing 2 to RHS
⇒ n = 1280/2
⇒ n = 640
Thus, the number of terms of odd numbers from 9 to 1287 = 640
This means 1287 is the 640th term.
Finding the sum of the given odd numbers from 9 to 1287
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 1287
= 640/2 (9 + 1287)
= 640/2 × 1296
= 640 × 1296/2
= 829440/2 = 414720
Thus, the sum of all terms of the given odd numbers from 9 to 1287 = 414720
And, the total number of terms = 640
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 1287
= 414720/640 = 648
Thus, the average of the given odd numbers from 9 to 1287 = 648 Answer
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