I would work out how fast cluster 1 is moving relative to cluster 2, which is 87,000kms^-1.
And then i simply do time = distance/speed = 1.149E15.
So assuming I've gotten that right, i have to apply the relativistic factor somewhere?
Homework Statement
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 traveling at 258,000 kms^-1 relative to the Earth
while cluster 2 is traveling in the same direction at 171,000 kms^-1 ...