When 15 is included in a list of natural numbers, their mean is increased by 2. When 1 is included in this new list, the mean of the numbers in the new list is decreased by 1. How many numbers were there in the original list?
A. 4
B. 5
C. 6
D. 8
Answer: Option A
Solution(By Examveda Team)
Let there be n numbers in the original list and let their mean be x.Then, sum of n numbers = nx
$$\eqalign{ & \therefore \frac{{nx + 15}}{{n + 1}} = x + 2 \cr & \Rightarrow nx + 15 = \left( {n + 1} \right)\left( {x + 2} \right) \cr & \Rightarrow nx + 15 = nx + 2n + x + 2 \cr & \Rightarrow 2n + x = 13.....(i) \cr} $$
And,
$$\eqalign{ & \therefore \frac{{nx + 16}}{{n + 2}} = \left( {x + 2} \right) - 1 \cr & \Rightarrow nx + 16 = \left( {n + 2} \right)\left( {x + 1} \right) \cr & \Rightarrow nx + 16 = nx + n + 2x + 2 \cr & \Rightarrow n + 2x = 14.....(ii) \cr} $$
Solving (i) and (ii), we get:
n = 4, x = 5
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