Because capacitors A, B and C are connected together in series, they all have the same charge. Since the charge of capacitor C is 3C, the charges of both of the capacitors A and B are also 3C.
The three-capacitor network containing capacitors A, B and C is equivalent to the capacitor on the left labeled ‘ABC’. The word ‘equivalent’ means that the voltage, charge and capacitance of capacitor ABC are equal to the voltage, charge and capacitance of the three-capacitor series network. In particular, the 3C charge on each of the capacitors of the three-capacitor network must be the same as the charge of capacitor ABC. The charge of the equivalent capacitor ABC is also 3C.
Capacitors A and B are connected together in series. Together, they are equivalent to the single capacitor on the left labeled ‘AB’. The word ‘equivalent’ means that the voltage, charge and capacitance of capacitor AB are equal to the voltage, charge and capacitance of the two-capacitor series network. In particular, the 5C of charge on capacitor AB must equal the charge on capacitor A as well as on B. The charges on both capacitors A and B are 5C.