Because resistors A, B and C are connected together in parallel, they all have the same voltage. Since the voltage across resistor A is 4V, the voltages across both of the resistors B and C are also 4V.
The three-resistor network containing resistors A, B and C is equivalent to the resistor on the left labeled ‘ABC’. The word ‘equivalent’ means that the voltage, current and resistance of resistor ABC are equal to the voltage, current and resistance of the three-resistor parallel network. In particular, the 4V across each of the resistors in the three-resistor network must be the same as the voltage across resistor ABC. Thus, the voltage across resistor ABC is also 4V.
Resistors A and B are connected together in parallel. Together, they are equivalent to the single resistor on the left labeled ‘AB’. The word ‘equivalent’ means that the voltage, current and resistance of resistor AB are equal to the voltage, current and resistance of the two-resistor parallel network. In particular, the 3V voltage across resistor AB must equal the voltage across resistors A as well as across resistor B. Thus the voltages across both resistors A and B are 3V.