Shortcut Trick
Each even-placed multiple is exactly 5 more than its preceding odd-placed multiple.
Multiples: (5, 10), (15, 20), (25, 30), ... (495, 500).
Difference between every pair (Even − Odd) = 5.
Therefore, Average of Even (Q) − Average of Odd (P) = Constant difference = 5.
∴ The correct answer is 5.
Alternate Method
Given: First 100 multiples of 5: {5, 10, 15, 20, ..., 500}
Formula Used: Average of an Arithmetic Progression (AP) = (First term + Last term) ÷ 2
⇒ Set P (Odd places): 1st, 3rd, ..., 99th multiples
⇒ P = {5, 15, 25, ..., 495}
⇒ Average P = (5 + 495) ÷ 2 = 500 ÷ 2 = 250
⇒ Set Q (Even places): 2nd, 4th, ..., 100th multiples
⇒ Q = {10, 20, 30, ..., 500}
⇒ Average Q = (10 + 500) ÷ 2 = 510 ÷ 2 = 255
⇒ Difference = Q − P = 255 − 250 = 5
∴ The correct answer is 5.
Additional Information
Average of Arithmetic Progression
For any sequence in AP, the average is always the middle term or the mean of the first and last terms: (a + l) ÷ 2.
Consecutive Terms Difference
If every term of set B is 'k' more than the corresponding term of set A, then Average(B) − Average(A) = k.
Sum of First n Multiples
Sum of first n multiples of 'x' = x × [n(n+1) ÷ 2]. The average is x × (n+1) ÷ 2.