Numbers of boys and girls are 'x' and 'y' respectively ; average ages of a girl and a boy are 'a' years and 'b' years respectively. The average age (in years) of all boys and girls.
Number of boys = $$x$$
Average age of boys = $$b$$
=> Total age of boys = $$bx$$
Similarly, total age of girls = $$ay$$
$$\therefore$$ Average age of all boys and girls = Total age of boys and girls / Number of boys and girls
=> Ans - (B)
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