Small tug boats pull cruise ships up and down rivers to prevent them from grounding. Cruise ships anchor 18 km away from the river port. A tug boat can travel 12 km downstream in the same time it takes it to travel 8 km upstream. If the speed of the current is 6 km/h, calculate the travel time it takes for the tug boats to go upstream from the cruise ship to the river port and then downstream back to the cruise ship.

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