Betty and Kevin finish watching a movie in the cinema and want to go home. Betty leaves earlier than Kevin and arrives at a grocery store in $$6$$ minutes. At this time, Kevin has walked $$150$$ metres. If both continue walking for another $$3$$ minutes at their previous speeds, Kevin will arrive at the store, and Betty will be $$900$$ metres away from the cinema. Find how many minutes earlier Betty leaves than Kevin does. 
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