A group of boys has an average weight of 36 kg. One boy weighing 42 kg leaves the group and another boy weighing 30 kg joins the group. If the average now becomes 35.7 kg, then how many boys are there in the group?
Let the number of boys in the group = $$x$$
Average weight of boys = 36
=> Total weight of $$x$$ boys = $$36x$$ kg
One boy weighing 42 kg leaves and another boy weighing 30 kg joins
=> New weight of the group = $$36x-42+30=(36x-12)$$ kg
$$\therefore$$ New average = $$\frac{(36x-12)}{x}=35.7$$
=> $$36x-12=35.7x$$
=> $$36x-35.7=12$$
=> $$0.3x=12$$
=> $$x=\frac{12}{0.3}=40$$
=> Ans - (C)
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