The solution to part (d) of Problem 4 is incorrect -- it gets the final numerical answer right only by coincidence. For such circuits, one can't just say that there's no resistance and apply energy conservation. Energy will always be lost through the wires. For example, consider charging a capacitor (capacitance C) with a battery (voltage E). To transfer charge q, The work done by the battery is qE, while the energy at the capacitor is q^2/2C = qE/2. As you can see, these don't match.