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In the above example, if the average weight is decreased by 1 $\mathrm{kg}$, then find the weight of new person.

Solution :

Let the average weight of a group $=x$

$\therefore$ Total weight of 30 persons $=30 x$

Let the weight of new person $=y$

$\therefore$ From the quetions,

\begin{aligned} & 30 x-40+y=30(x-1) \\ \Rightarrow & 30 x-40+y=30 x-30 \\ \Rightarrow & y=40-30 \\ \therefore & y=10 \mathrm{~kg} ; \text { Ans. } \end{aligned}

 Trick : The age of new person$$=40+30$$[New average - Old average]$$=40+30 \times(-1)=10 \mathrm{~kg} ; \text { Ans. }$$

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