There is a couple with their son and daughter and a dog going out on a trip. They have to cross a river on the way. There is an empty boat at the ferry, which can carry up to 50 kilograms. The couple weighs 50 kilograms each, and the son and daughter weigh 25 kilograms. 10 kg. Ask: How should they cross the river?
有夫妇带着儿子和女儿，一条狗外出旅行，途中要过一条河，渡口有一只空船，最多能载50千克，而夫妇二人各重50千克，儿子与女儿各重25千克，狗重10千克．请问：他们应该如何过河？
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