The boat can fit two children at one time but not two adults. Then we have the issue of it not specifying if they are all adults or two adults and two children or 1 adult with three children. so solution have a child and adult go together. with a child in the adults lap. while it is not safe it is possible (warning this is a answer that could irritate any teacher with sense) or presuming this is a family of two adults ad two children, send the two children first together then send the adults separate 3 trips in total