1. The problem statement, all variables and given/known data The two plates of a parallel-plate capacitor initially carry equal amount of positive charge. If some charges are transferred from one plate to another, the charges on the plates are respectively +900uC and +100uC. The potential difference across the plates becomes 4V. What is the capacitance of the capacitor? 2. Relevant equations C=Q/V 3. The attempt at a solution I just pick randomly pick +900uC instead of +100uC: C=900/4=225uC 4. Then I check the answer: Totally 400uC moves away from one to the metal plate and move to another plate, so: C=400/4=100uC According to what I have learn, Q is the charge of one of the metal plate. In here, why should I subsitute 400uC, the amount of charge that is transferred?