The mean of 1^2,2^2,3^2,4^2,5^2,6^2,7^2 is?  Choose one:
A) 15  B) 18  C) 20  D) 25  E) 30
1^2+2^2+3^2+-----+n^2 = n(n+1)(2n+1)/6   1^2+2^2+3^2+-----+7^2 = 7*8*156 = 140   Required average is = 140/7= 
<gadget id="calculator">140/7</gadget>
<output>20</output>
20   Answer is 20
<result>C</result>
The mean of 1^2,2^2,3^2,4^2,5^2,6^2,7^2 is?