In calculating the mean of 8 numbers, a boy mistakenly used 17 instead of 25…

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. 

Correct Answer: Option C

Explanation

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