Given here is an online R1 R2 Calculator for calculating R1 R2 from Ze Zs. R1 is the resistance of circuit phase conductor and R2 is the resistance of circuit earth conductor. Loop Impedance (Ze) is the External Impedance of earth fault current path (loop) to Earth. Direct Measurement (Zs) is the total Impedance of earth fault current path (loop) to Earth. You can calculate R1 + R2 (i.e) measured value based on the direct measurement and loop impedance.
Calculate the Resistance (R1+R2) for direct measurement (Zs) as 15 and loop impedance (Ze) as 12.
R1 + R2 = Zs - Ze
= 15 - 12