In calculating the mean of 8 numbers, a boy mistakenly used 17 instead of 25 as one of the numbers. If he obtained 20 as the mean, find the correct mean.
Let the sum of the 8 numbers = y.
\(\frac{y}{8} = 20 \implies y = 20 \times 8 = 160\)
\(160 - 17 = 143 (\text{the sum of the other 7 numbers})\)
\(mean = \frac{143 + 25}{8} = \frac{168}{8}\)
= 21
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