If the Milky Way and the Andromeda galaxies are approximately 2 million light years apart and are moving toward each other at 199 km/sec, how long will it take before the two galaxies collide?
A light year is a unit of length equal to 9.46•10^15 m
t = s/v = 2•10^6•9.46•10^15/199000= 9.5•10^16 s= 3•10^9 years