1. The problem statement, all variables and given/known data You observe that in a rich cluster of radius 2 Mpc, galaxies move with a typical speed of 1500km s^(-1). Estimate the mass of the cluster. 2. Relevant equations I can't seem to find any type of formula that makes sense to me in regards to this question. 3. The attempt at a solution I don't even know where to begin. I though of using a simple mass formula, but I can't find one that uses speed and radius as givens.