
In the circuit, the resistance of arm ACB (30 + 30 = 60 Ω) is the parallel with the resistance of arm AB (= 30 Ω).
Hence, the effective resistance of the circuit is

Common Errors and Its Rectifications:
Common Errors:
1. Sometimes students think that charge and current are same.
2. In doing calculation part students can’t give the importance to mention the units.
3. They may confuse the parallel and series network of the resistance.
Rectifications:
1. Charge q = ne Current I = q/t
2. Unit is very importance to the every physical quantities.
3. If the resistors are series, their resultant is sum of the all the reciprocal of individual resistance. If the resistors are parallel their resultant is sum of the individual resistance