1.

The mean of 9 observations is 36. If the mean of the first 5 observations is 32 and that of the last 5 observations is 39, then the fifth observation is ....(A) 28 (B) 31 (C) 43 (D) 37

Answer»

The correct option is: (B) 31 

Explanation:

36 x 9 = x1 + x2 + x3 +...+x9      ...(i) 

5 x 32 = x1 + x2 + x3 + x4 + x5    ...(ii)

5 x 39 = x5 + x6 + x7 + x8 + x    ...(iii)

Adding (ii) and (iii), we get 

355 = x1 + x2 + x3 + x4 + x5 + ... + x9 + x5

Using (i), we get 355 - 36 x 9 = x5 => x5 = 31



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