Rockfall (Planetary Crossings Part III)

Harassed during important experiments, three Earthlings were taking three law-breaking Grundi to the authorities. Suddenly they heard the thunderous roar of a rockfall, and they knew they were facing sure death unless they crossed to the other side of the canal. The portable watercraft they carried with them would hold only two passengers, regardless of weight. At no time could there be more Grundi on either side of the water than Earthlings or the Grundi would overpower the Earthlings. How could they all cross the water safely?

(Source: Fantastic Book of Logic Puzzles by Muriel Mandell, illustrated by Elise Chanowitz)

The secret to solving this one is the somewhat baffling condition that any number of Grundi can be on one side of the canal without chaperoning; they will not get away.

To begin with, sending only Grundi will not get anywhere and sending only Earthlings is not permissible. So send one Earthling and a Grundi to the far side. From there, the only way to make progress is sending one Earthling back. From there, send both Grundi on the near side over, but send one back. Then send both Earthlings to the far side, because there are two Grundi there and only one Earthling would get overpowered. Then send one Earthling and one Grundi to the near side. Lastly, send both Earthlings to the far side. This will result in two Grundis on the near side with one Grundi and all three Earthlings on the far side. Then send one Grundi back to the near side, and then bring two to the far side. Lastly, it is possible to send one Earthling over to the near side to bring the remaining Grundi over to the far side.