There is a river that is $4$ meters across and makes a $90^\circ$ turn. Is it possible to cross the river by bridging it with only two planks, each $3.9$ meters long?
So the shape of the river is like a thick letter 'L'. The planks are $0.1$ meters too short to simply cross the river with one of them. Also I don't see how to use the other plank.
On
When you put the planks over the bend, you create a triangle, the four metres across the river directly, then the four metres up the river. The length of planks needed therefore is $\sqrt{4^2+4^2}=4\sqrt2\approx 5.6m$
Since $3.9×2=7.8m$, you easily have enough plank to cross. One method would be to bolt the planks together so the amount of plank that is layered is at most 1.7m, or you could leave the first plank with 2.1m on land and 1.8m off land, then carry the other plank and lay it between the other shore and the plank, then walk the rest.
Yes, it is possible. The planks are GH and EI. $BG=BH = \frac {3.9}{\sqrt 2}$