r123=R*3=6*3=18 Ом
r1234=R*r123/(r123+R)=6*18/(6+18)=4.5 Ом
r123456=r1234+R+R=4.5+6+6=16.5 Ом
r1234567 = r123456*R/(R+r123456)=16.5*6/(16.5+6)=4.4 Ом
r123456789=r1234567+R+R=4.4+6+6=16.4 Ом
r12345678910=R*r123456789/(R+r123456789)=16.4*6/(6+16.4)=4.39 Ом
Rob=r12345678910+R+R=4.39+6+6=16.39 Ом