Three is an infinite wire grid with square cells. The resistance of each wire between neighboring joint connection is equal to R. If the resistance of the whole grid between points A and B is nR then 2n is…………. 

Suppose that the voltage V is applied between the points A and B, then $V=IR=I_0{R}_0$
Where Ri s the resistance of the whole grid. I is the current through the grid and $I_0$  is the current through the segment AB. Now from symmetry I/4 is the part of the current flowing through all the four wire segment, meeting at the point A and similarity the amount of current flowering through the wires meeting at B is also I/4. Thus a current I/2 flows through the conductor AB i.e.’ $I_0=\frac{I}{2}$
Hence $R=\frac{R_0}{2}$