Let 9 = x1 < x2 < …< x7 be in an A.P. with common difference d. If the standard deviation of x1, x2 …, x7 is 4 and the mean is ˉ x , then ˉ x + x6 is equal to:
(1) 18(1 + 1/√3)
(2) 34
(3) 2(9 + 8/√7)
(4) 25