The distance between Town A and Town B is $620\text{km}$. Eric drives from Town A to Town B at a speed of $$75\text{km/h}$$ while Chien drives from Town B to Town A at a speed of $$80\text{km/h}$$. How many hours will it take for them to meet each other? 
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As they are driving toward each other, the total speed is $$75+80=155$$ miles per hour. Then the time needed is $$620\div155=4$$ hours. 