“A $3\ \mu\text{F}$ capacitor and a $6\ \mu\text{F}$ capacitor are in series across a $30\ \text{V}$ battery. The two plates at the joint carried $+90\ \mu\text{C}$ together before the circuit was built. What is the potential of the joint, measured from the far plate of the $6\ \mu\text{F}$ capacitor?” is available to Standard subscribers.