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MCQs Math


Question:     Find the average of odd numbers from 5 to 643


Correct Answer  324

Solution And Explanation

Solution

Method (1) to find the average of the odd numbers from 5 to 643

Shortcut Trick to find the average of the given continuous odd numbers

The odd numbers from 5 to 643 are

5, 7, 9, . . . . 643

After observing the above list of the odd numbers from 5 to 643 we find that the difference between two consecutive terms are equal. This means the list of the odd numbers from 5 to 643 form an Arithmetic Series.

In the Arithmetic Series of the odd numbers from 5 to 643

The First Term (a) = 5

The Common Difference (d) = 2

And the last term (ℓ) = 643

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 5 to 643

= 5 + 643/2

= 648/2 = 324

Thus, the average of the odd numbers from 5 to 643 = 324 Answer

Method (2) to find the average of the odd numbers from 5 to 643

Finding the average of given continuous odd numbers after finding their sum

The odd numbers from 5 to 643 are

5, 7, 9, . . . . 643

The odd numbers from 5 to 643 form an Arithmetic Series in which

The First Term (a) = 5

The Common Difference (d) = 2

And the last term (ℓ) = 643

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 5 to 643

643 = 5 + (n – 1) × 2

⇒ 643 = 5 + 2 n – 2

⇒ 643 = 5 – 2 + 2 n

⇒ 643 = 3 + 2 n

After transposing 3 to LHS

⇒ 643 – 3 = 2 n

⇒ 640 = 2 n

After rearranging the above expression

⇒ 2 n = 640

After transposing 2 to RHS

⇒ n = 640/2

⇒ n = 320

Thus, the number of terms of odd numbers from 5 to 643 = 320

This means 643 is the 320th term.

Finding the sum of the given odd numbers from 5 to 643

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 5 to 643

= 320/2 (5 + 643)

= 320/2 × 648

= 320 × 648/2

= 207360/2 = 103680

Thus, the sum of all terms of the given odd numbers from 5 to 643 = 103680

And, the total number of terms = 320

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 5 to 643

= 103680/320 = 324

Thus, the average of the given odd numbers from 5 to 643 = 324 Answer


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(5) Find the average of odd numbers from 15 to 31

(6) Find the average of even numbers from 8 to 352

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