Resistors A, B and C are connected together in series. Together, they are equivalent to the single resistor on the left labeled ‘ABC’. The resistance of the entire three-resistor network is equal to the sum of the resistances across the individual resistors:
RABC = RA + RB + RC.
where RABC, RA, RB and RC are the resistances of resistors ABC, A, B and C respectively. Substituting in the values from the diagram gives:
RABC = 3Ω + 4Ω + 5Ω = 12Ω.
Resistors A, B and C are connected together in series. Together, they are equivalent to the single resistor on the left labeled ‘ABC’. The resistance of the entire three-resistor network is equal to the sum of the resistances of the individual resistors:
RABC = RA + RB + RC.
where RABC, RA, RC and RC are the resistances of resistors ABC, A, B and C respectively. Substituting in the values from the diagram gives:
16Ω = RA + 4Ω + 5Ω.
Solving for RA reveals that RA = 7Ω.