The average of seven numbers is 18. If one of the number is 17 and if it is replaced by 31, then the average becomes :
Options:
A . 19.5
B . 21.5
C . 21
D . 20
Answer: Option D Answer: (d)Difference = 31 – 17 = 14 ∴ Required average = 18 +$14/7$ = 20 Aliter : Using Rule 26,If average of n numbers is m later on it was found that a number 'a’ was misread as 'b’. The correct average will be = m +${\text"(a-b)"}/ \text"n"$.Here, n = 7, m = 18 a = 31, b = 17 New Average = m + ${\text"(a - b)"}/\text"n"$= 18 +${(31 – 17)}/7$= 18 +$14/7$= 18 + 2 = 20
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