Question 1178799
<pre>
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</pre>