The distance between port a and port B is 1200km. Ship a sails from port a to port B at 60km per hour. After 30 minutes of departure, ship B sails from port B to port a at 70km per hour. The two ships meet a few hours after departure

（1200-60x0.5）÷（70+60）
＝1170÷130
= 9 hours
A 9-hour meeting

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