1. The problem statement, all variables and given/known data You should take the velocity of light c = 3 x 105 km s^-1 Two spherical clusters of galaxies each have a diameter D = 10^20 km. Cluster 1 is travelling at 258,000 kms^-1 relative to the Earth while cluster 2 is travelling in the same direction at 171,000 kms^-1 . Suppose that cluster 1 is just starting to pass through cluster 2 in the reference frame of the Earth. How long would it take for cluster 1 to pass completely through cluster 2 in the reference frame of the Earth, assuming that there is no gravitational interaction between the two galaxy clusters. Passing time = ? x D / c Enter your answer to ONE decimal place as a multiple of D/c, i.e. without entering values for D/c at this stage Now evaluate your answer in millions of years. = ? million years Enter your answer to the nearest integer. 2. Relevant equations 3. The attempt at a solution I've not done relativity since I was at college over a year ago, and now i have some coursework that is all on relativity and I have no idea what i'm doing, so any help or guidance would really be appreciated, thanks.