A metal wire PQ of mass 10 g lies at rest on two horizontal metal rails separated by 5 cm as shown in figure. A vertically downward magnetic field of magnitude 0.80 T exists in the space. The resistance of the circuit is slowly decreased and it is found that when the resistance goes below 20.0 Ω, the wire PQ starts sliding on the rails. The coefficient of friction between wires and rails is found to be n/25. Find n.