Two people, each with a mass of 30.0 kg, stand on opposite sides of a horizontal 3.0m-long 10.0-kg log in a still pond. There is a big red mark painted at the exact center of the log's length (i.e., between the two people). Initially, the people and the log are at rest relative to the shore. Then, one person joins the other by walking to the other end of the log (so now there are two people at the same edge of the log; assume that the log does not tip over). Ignoring any frictional resistance between the pond water and the log, what distance has the red mark on the log moved, relative to the shore? For credit, you must show a center-of-mass calculation