Delhi Police Constable Exam Model Paper 10

Tips & Tricks
Given:
Average of certain first natural numbers is 20.5
Formula used:
Average = Sum of observations/ number of observations
Average of ‘first’ “n” natural numbers = [n + 1]/2
Calculation:
Average of certain first natural numbers $=20.5=\frac{\left[\mathit{n}+1\right]}{2}$
⇒ $\mathit{n}+1=41$
⇒ $\mathit{n}=40$
∴ Sum of these first 40 natural numbers = average × number of observations $=20.5\times 40=820$