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In calculating the mean of 8 numbers, a boy mistakenly us...

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. 

A.

24

B.

23

C.

21

D.

19

Correct answer is C

Let the sum of the 8 numbers = y.

y8=20y=20×8=160

16017=143(the sum of the other 7 numbers)

mean=143+258=1688

= 21