The number of boys that leave the room must equal the number of boys that remain. Since $$5$$ boys leave, there are $$5$$ boys still in the room for a total of $$10$$ boys. 
There are twice as many boys in a room as girls. If $$5$$ boys leave the room, there would be an equal number of boys and girls in the room. How many boys were in the room at first? 

$$5$$ 
$$10$$ 
$$15$$ 
$$20$$ 