Shortcut Trick
For consecutive natural numbers, the average is always the middle term of the series.
For 23 numbers, the middle term is the 12th number.
The last 11 numbers are from the 13th to the 23rd position; their middle term is the 18th number.
Given 18th number = 34 ⇒ 12th number = 18th number − (18 − 12) = 34 − 6 = 28.
∴ The correct answer is 28.
Alternate Method
Given: Total numbers = 23; Numbers are consecutive natural numbers in ascending order; Average of last 11 numbers = 34.
Formula: Average of an Arithmetic Progression (AP) = (First term + Last term) ÷ 2
Let the 23 consecutive numbers be x, x+1, x+2, ..., x+22.
⇒ The last 11 numbers are: (x+12), (x+13), ..., (x+22).
⇒ Average of last 11 = [(x + 12) + (x + 22)] ÷ 2
⇒ 34 = (2x + 34) ÷ 2
⇒ 34 = x + 17
⇒ x = 34 − 17 = 17
⇒ Average of all 23 numbers = [First term + Last term] ÷ 2
⇒ Average = [x + (x + 22)] ÷ 2 = x + 11
⇒ Average = 17 + 11 = 28
∴ The correct answer is 28.
Additional Information
Average of Arithmetic Progression
For any series in AP, the average is always the mean of the first and last terms: Average = (a + l) ÷ 2.
Middle Term Rule
If the number of terms (n) is odd, the average is the (n + 1)/2th term. If n is even, it is the mean of the two middle terms.
Sum of Natural Numbers
The sum of the first n natural numbers is given by the formula S = [n × (n + 1)] ÷ 2.