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I see this question in "An introduction to modern cosmology - Andrew Liddle - Wiley Publication":

In the real Universe the expansion is not completely uniform. Rather, galaxies exhibit some random motion relative to the overall Hubble expansion, known as their pecuLiar veLocity and caused by the gravitational pull of their near neighbours. Supposing that a typical (eg root mean square) galaxy peculiar velocity is 600 km/s, how far away would a galaxy have to be before it could be used to determine the Hubble constant to ten per cent accuracy, supposing

(a) The true value of the Hubble constant is 100 km/s Mpc?

(b) The true value of the Hubble constant is 50 km/sMpc ?

Assume in your calculation that the galaxy distance and red-shift could be measured exactly. Unfortunately, that is not true of real observations.

I really don't know how to start solving this. What do we need to find Hubble constant. I know $v=H.r$ but I don't know how to calculate the effect of peculiar velocity. we don't know $v$ and $r$ in Hubble formula, how to find $H$?

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You can start by working out how fast a galaxy must be receding if its peculiar velocity (given as about 600 km/s) is less than or equal to 10% of its expansion velocity.

I'm not going to spell out the next step as you said you don't know where to start.

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