The average of a collection of $ 20$ measurements was calculated to be $56$ cm. But later it was found that a mistake occured in one of the measurements which was recorded as $ 64$ cm but should have been $ 61$ cm. What is the correct average?

The original sum of the 20 measurements is 20 times 56, which equals 1120 cm. The correct sum decreases because the recorded measurement of 64 cm should be 61 cm, so the actual sum is 1120 minus 3, giving 1117 cm. The correct average is calculated by dividing this corrected sum by 20. The correct average is 55.85 cm.

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