Answer :
Mean of first n natural numbers = (n+7)/3
⇒[tex] \frac{1+2+3+4+.......+n}{n}= \frac{n+7}{3} \\ or, \frac{n}{2n} . [2*1+(n-1).1] = \frac{n+7}{3} [/tex]
[tex] \frac{n+1}{2} = \frac{n+7}{3} \\ or,3n+3=2n+14[/tex]
or, 3n-2n = 14-3
or, n = 11
⇒[tex] \frac{1+2+3+4+.......+n}{n}= \frac{n+7}{3} \\ or, \frac{n}{2n} . [2*1+(n-1).1] = \frac{n+7}{3} [/tex]
[tex] \frac{n+1}{2} = \frac{n+7}{3} \\ or,3n+3=2n+14[/tex]
or, 3n-2n = 14-3
or, n = 11