Four Adventurers and a Small Canoe Puzzle

The Puzzle:

Four adventurers (Alex, Brook, Chris and Dusty) need to cross a river in a small canoe.

The canoe can only carry 100kg.

Alex weighs 90kg, Brook weighs 80kg, Chris weighs 60kg and Dusty weighs 40 kg, and they have 20kg of supplies.

How do they get across?

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