Divide Paper into Fifths: Fujimoto Approx.
Interestingly, you can "eye-ball" a 1/5 division and get surprisingly good results if you employ Fujimoto's Approximation. This method decreases the initial error by a half every time you make a fold. Although it is an approximation, the final result is often good enough to proceed with your origami folding.
- Start on the left-edge of the paper and make a pinch fold at a location which you estimate to be approximately 1/5th of the length of the paper.
Call this pinch mark "A".
- Fold the right-corner of the paper so it meets with "A". Make pinch fold "B".
- Fold the right-corner of the paper so it meets with "B".
Make a pinch fold "C".
- Fold the left-corner of the paper to meet with "C" and make pinch fold "D".
- Fold the left-corner of the paper to pinch mark "D". Press down to form crease "E". Assuming that your initial pinch "A" is not too far off, crease E will be quite close to the true 1/5th division.
- At this point, it is just a matter of folding the right side of the paper in half to get 2/5th segments.
These are then folded in half to get five 1/5th divisions.
Fujimoto's approximation works for any divisions and any angle too. The mathematics behind the Fujimoto Approximation is discussed in Tom Hull's book Project Origami: Activities for Exploring Mathematics. You can also learn more about this approximation here.