Explanation:

Let the Weight of n girls=x and weight of m boys =y
As per the question, $$n=60\times \dfrac{40}{100}=24$$
Given that $$m+n=60$$
$$m=60-24=36$$
Average weight of boys $$\dfrac{y}{36}=62$$Kg
$$\Rightarrow y=62\times 36$$
Average weight of girls $$\dfrac{x}{24}=55$$Kg
$$\Rightarrow x=55\times 24$$
So, average weight of whole classes $$=\dfrac{x+y}{m+n}=\dfrac{62\times 36 +55\times 24}{24+36} $$
$$\Rightarrow \dfrac{2232+1320}{24+36}=\dfrac{3553}{60}=59.2$$Kg