To cross this river 6 meters wide, these two hikers only have two boards of 5 meters each. How can they pass without getting wet?
A plank with 3 meters of its length is placed on land and the other two on the river (like a trampoline on the water). One of the hikers is located at the end of the plank that is on the mainland as a counterweight. The other hiker, carrying the other board can walk smoothly to the other end of the first board. In that place extend the second plank to the other shore that is 2 meters from its position so that it places 1/2 meter of the plank on top of the other plank and 1/2 meter on the next shore. The remaining 4 meters is just the distance left to cross the river. With that plank placed you can walk to the opposite shore.
Once reached the other shore you can pick up your board and do the reverse process for your partner to pass. That is, it will place it 3 meters inland with 2 meters that will remain on the water and placed at the end of the plank that is on the mainland. Then, your partner, from the initial shore will place his own plank with 1/2 meter on the shore and 1/2 meter on the board of his partner which will allow him to cross the river as well.