If the arithmetic mean of 13 observations 511, 512, 514, 505, 513, 507, 509, 504, 512, 513, X, 507, 511 is 492. Find the value of "X" in the observations.

Correct option is A

Mean = 492
Number of observations = 13
Total sum = 13 × 492
= 6396

Sum of known numbers:
511 + 512 + 514 + 505 + 513 + 507 + 509 + 504 + 512 + 513 + 507 + 511
= 6118

Let unknown value = X
6118 + X = 6396
X = 6396 − 6118
X = 278