Discussion

Explanation:

Mean of 100 observations = 60

Correction to be made = x – 80

New average, 60×100 + (x - 80)100 = 59.5

⇒ 6000 + x – 80 = 5950

⇒ x = 30.

Alternately,

Since the average of 100 numbers decreases by 0.5, the sum decreases by 100 × 0.5 = 50.

Hence the number added is 50 less than the number removed

⇒ 50 = number removed – number added = 80 – x.

⇒ x = 30.

Hence, option (c).

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