Find the positive value of x if the standard deviation of the numbers 1, x + 1 is 6

A.
1 
B.
2 
C.
3 
D.
4
Correct Answer: Option C
Explanation
mean (x) = (frac{1 + x + 1 + 2x + 1}{3})
= (frac{3x + 3}{3})
= 1 + x
(begin{array}{cc} X & (X X) & (X X)^2\ hline 1 & x & x^2 \ x + 1 & 0 & 0\2x + 1 & x & x^2\ hline & & 2x^2end{array})
S.D = (sqrt{frac{sum(x – 7)^2}{sum f}})
= (sqrt{(6)}^2)
= (frac{2x^2}{3})
= 2x^{2}
= 18
x^{2} = 9
∴ x = (pm) (sqrt{9})
= (pm)3