The mean of 12, 22, 32, 42, 52, 62, 72 is -

Answer: Option B

1² + 2² + 3² + . . . . . + n² = $$\frac{n\left(n+1\right)\left(2n+1\right)}{6}$$
∴ 1² + 2² + 3² + . . . . . + 7² = $$\left(\frac{7\times 8\times 15}{6}\right)$$   = 140
So, required average
$$=\left(\frac{140}{7}\right)$$
= 20