I need help with on getting the idea for the following question found in 4b AQA PHYA 2A 2013 (Astrophysics):
4b) Ursa Minor also contains the galaxy NGC 6251. Measurements indicate that the lightfrom the galaxy has a red shift, z, of 0. 025 and that the galaxy is 340 million light yearsfrom Earth.
4bi) Use these data to calculate a value for the Hubble constant.
TL;DR In the mark scheme, where did the value 3x10^5 come from?
And actually passed?