Alex, Brook, Chris, and Dusty need to cross a river in a small canoe. The canoe can carry only 100 kg. Alex weighs 90 kg, Brook weighs 80 kg, Chris weighs 60 kg, Dusty weighs 40 kg, and they have 20 kg of supplies.
Find the minimum number of times they have to row across the river so that everyone and the supplies are safely across, assuming that at least one person must be in the rowboat during the trips.
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