The average of 6 observations is 13. A new observation is included and the new average is decreased by 1. The seventh observation is?
Pick one.
A) 1  B) 3  C) 5  D) 6  E) 7

Rationale: let seventh observation = x . then , according to the question we have = > ( 78 + x ) / 7 = 12 = > x = 6 . hence , the seventh observation is 6 . answer : d
Answer: D