Down the stairs: Xiaoding, Xiaoming and Xiaohong walk down the dusty stairs side by side from the top. Xiaoming takes 2 steps, Xiaohong takes 3 steps and Xiaoding takes 4 steps. If there are three people's footprints at the top and bottom of the building, how many steps are there on the stairs with only one person's footprint? Analysis: since everyone walks downstairs from the top of the building, the order of the stairs must be the common multiple of the order of each step taken by three people. The least common multiple of 2, 3 and 4 is 12, so this staircase has at least 12 steps. Only one person's footprint is Step 2, Step 3, Step 9, Step 10. Because these numbers only contain one factor of 2, 3 and 4. So there are at least four stairs with only one person's footprint.