There are $$30$$ students in your class. The ratio of boys to girls is $$3:2$$. How many boys and girls are there?. 
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According to the ratio $$3:2$$, we could know the total is $$5$$. Then the number of boys is $$30\times \dfrac{3}{5} =18$$.The number of girls is $$30\times\dfrac{2}{5}=12$$. 