Lower resistance on extreme left and upper resistance on extreme right are ineffective. The resistance
R2 and
R3 are in series combination. Therefore their equivalent resistance,
R′=R2+R3 =10+10=20Ω Similarly, the resistance
R5 and
R6 are in series combination. Therefore their equivalent resistance,
R"=R5+R6 =10+10=20Ω Now the equivalent resistances
R′ and
R" are in parallel combination. Therefore their equivalent resistance,
R"′= ===10Ω Thus equivalent resistance between
A and
D,
R=R1+R"′+R4 =10+10+10=30Ω(∵ series combination
)