The average of a collection of 20 measurements was calculated

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The average of a collection of 20 measurements was calculated to be 56 cm. But later it was found that a mistake had occurred in one of the measurements which was recorded as 64 cm., but should have been 61 cm. The correct average must be

1. 53 cm
2. 54.5 cm
3. 55.85 cm
4. 56.15 cm

Correct Option: C

Total length of 20 measurements = 56 × 20 = 1120 cm
Correct length of 20 measurements = 1120 – 64 + 61 = 1117
Correct average = 1117 ÷ 20 = 55.85 cm
Second method to solve this question with the help of given formula :
Here, n = 20, m = 56 , a = 61, b = 64

Correct Average = m +	(a - b)
	n

Correct Average = 56 +		61 - 64
		20

Correct Average = 56 -	3
	20

Correct Average = 56 – 0.15 = 55.85 cm