Question

If the sum of the first n natural numbers is 1/5 times the sum of the their squares, then the value of n is

• A. 5
• B. 6
• C. 7
• D. 8

Answer 1

Answer: (C)

7

$\frac{n(n+1)}{2}=\frac{1}{5}\left(\frac{n(n+1)(2n+1)}{6}\right)\Rightarrow 15=2n+1\Rightarrow n=7$