申論題

2. A research vessel performs a river survey. The vessel moves downstream with a constant speed 6 m/s relative to the water. The river has a steady current of 1 m/s relative to the ground. Mary sits on the boat and his instrument falls into the river. Five minutes later, she notices that his instrument is missing and immediately turns the boat around, moves upriver with the same speed of 6 m/s relative to the water. How long does it take the man to row back upriver to reclaim his instrument ? (20%)