## Perfect Ruler

#### Problem

By marking off six unit lengths on a piece of wood, we can make a ruler to measure all the lengths from 1 to 6 units directly.

But it is possible to achieve the same thing by using just two marks.

What is the minimum number of marks necessary to measure all the lengths from 1 to 12 units?

Problem ID: 70 (Mar 2002) Difficulty: 2 Star