The mean of 7 observations is 20. If the mean of the first 4 observations is 12 & that of the last 4 observations is 28, find the 4^th observation.

Since the mean of 7 observations = 20

Now, Total of 7 observation = 20 x 7 = 140
Mean of first 4 observations = 12
Total of the first 4 observations = 12 x 4 = 48
Mean of first 4 observations = 4 x 28 = 92
Total of first 4 observations =26
Total of 7 observations + 4^th observation = 48 + 92
=> 140 + 4^th observation = 140
=>  4^th observation = 140 – 140 = 0