Four Adventurers and a Canoe

 
Four Adventurers and a Canoe

Problem statement:

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?

Done thinking? Click here to view the solution.

Chris and Dusty row across, Dusty returns.
Alex rows over, and Chris returns.
Chris and Dusty row across again, Dusty returns.
Brook rows across with the Supplies, and Chris returns.
Chris and Dusty row across again for the last time.
 
(Note: some variations on this are possible)