Applet by Klaus Nagel
To construct a ruler which can measure all lengths from 1 through 29 with as few marks as possible. Hint: The minimum number of marks is 9. And there are (up to mirror symmetry) 6 rulers with this quality.
Ok., here is one example. Now find the 5 other ones! Good luck!
[0,1,4,10,16,22,24,27,29]