SOLUTION: Four people on one side of a river need to cross the river in a boat that can carry a maximum load of 180 pounds. The weights of the people are 80, 100, 150, and 170 pounds. Wh

The only time two can go across is when the 80 and 100 pounders
go together.

Here it is with 6 crossings:

 Not across    |   Across
--------------------------------
80,100,150,170 |   none across
       150,170 | 80,100             1st crossing
80,    150,170 |    100             2nd crossing 
80,        170 |    100,150         3rd crossing
80             |    100,150,170     4th crossing
80,100         |        150,170     5th crossing
  all across   | 80,100,150,170     6th crossing

Or it can be this way

 Not across    |   Across
--------------------------------
80,100,150,170 |   none across
       150,170 | 80,100             1st crossing
   100 150,170 | 80                 2nd crossing 
   100     170 | 80,    150         3rd crossing
   100         | 80     150,170     4th crossing
80,100         |        150,170     5th crossing
  all across   | 80,100,150,170     6th crossing

Either way it takes 6 crossings.

Edwin