Fat travelers

Another nice crossing puzzle, as seen at Maths is Fun.

Puzzle

Four travelers (Al, Bob, Chris, and Zoe) come to a river and find an old boat, with a sign reading "Weight limit: 300 lbs!" Unfortunately, Al weighs 270 lbs, Bob weighs 230 lbs, Chris weighs 180 lbs and Zoe weighs 120 lbs (as she is only 10). They also have 50 lbs of supplies. How can the fat travelers cross the river without sinking the boat?

References

Maths is Fun - math and logic puzzles appropriate for math education.