Nearby, you see a bunch of logicians, none of whom are very short. You can tell they're logicians because they're sitting around wearing different colored hats and not saying anything. You also notice that from the time you spot them in the distance to the time you've walked up next to them, none of them have blurted out the color of their hats. You therefore conclude that these logicians aren't exactly the brightest bulbs in the chandelier.
"Hey, can you guys give me a hand" you ask. One of the six logicians sitting outside snaps back, "Bah, we have no time for non-intellectuals like yourself. Bother neither us, nor the rest of our logician group who are inside our headquarters over there." You look toward the building he pointed to, and notice that one of the light bulbs is burned out. Getting an idea, you ask "Tell me, O logician, is the number of members in your group prime, or not?" He looks at you inquisitively and hesitantly replies "Our number is not prime." "Then there are eight among you" you state triumphantly.
Amazed at your deductive powers, the logicians offer to aid your quest to cross the river of lava. One of them offers: "We could dismantle our headquarters and use the planks of wood to build a bridge, although no single plank is long enough to make it across and we have no good means of fastening them together." You take one look at their termite-infested hovel and decline. You then notice a horrid smell and ask them what it is. "Oh, that" one replies "is our garbage heap. We could never deduce a fair way of allocating garbage disposal duties, so it just piled up outside."
NOW FOR THE PUZZLE: You find that each logician (and yourself) can pick up a pile of garbage and dump it somewhere in 1 minute. Each pile makes a 5 foot cube that can serve as a bridge over the lava. But once placed in the lava, such a cube will be consumed after 3 minutes.
The pile of available garbage is practically infinite for the purposes of this puzzle. The width of the river is 220 feet (= 44 cubes of size 5 feet). You have disposed of the logicians' hats, so you can enlist the aid of all eight of them at once. How would you get across? (There are actually TWO answers that I have in mind, one of which is very logically-mined.)
And finally, to get rid of some of the trivial solutions: