The average weight of a group of eight people increased by 2.5 kg when a person weighing 80 kg joined in place of one of the members of the group. The weight of the member replaced was:
given that the average weight of 8 people is increased by 2.5kg when a person replaced a group member
let the group member be z
average be k
sum be x
$$\frac{\left(x+80-z\right)}{8}=k+2.5$$
8k+80-z=8k+20 [here we know that x = 8k from the given information.]
k gets canceled on both sides
z=60 i.e; the weight of the replaced member is 60
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