The effective resistance between points A and B in the . . .

Question : The effective resistance between points A and B in the given circuit is :

(a) 6Ω

(b) 8/3 Ω

(c) 16/3 Ω

(d) 2Ω

Doubt by Japjee

Solution : 

P=[(4+2)3]/[(4+2)+3]

P=18/9

P=2Ω

Q=4Ω

P/Q = 2/4 = 1/2 

R=[(3+6)18]/[(3+6)+18]

R=[9×18]/27

R=6Ω

X=12Ω

R/X=6/12 = 1/2

Clearly 

The Bridge is balanced. So no current will pass through the 7 Ω resistor. 

R_eq=[(P+R)(Q+X)]/[(P+R)+(Q+X)]

R_eq=[(2+6)(4+12)]/[(2+6)+(4+12)]

R_eq=[(8)(16)]/[(8)+(16)]

R_eq=[8×16]/24

R_eq=16/3Ω