What is the value of unknown resistance R, of galvanometer shows null deflection in the given meter bridge set up shown in the fig?

Given that, the galvanometer shows null deflection. Hence the given combination is Wheatstone bridge.

Let $k$ be the resistance per unit length of the meter bridge wire.

Applying Wheatstone bridge principle we get:-

$\dfrac{20k}{55}=\dfrac{80k}{R}$

$\implies R= \dfrac{80\times 55}{20} =220\ \Omega$