4. (25 pts) Provide a design of the sequential circuit for the vending machine controller one product sells for $0.35, and the machine takes only quarters (input Q) and dimes (input D) with an output signal I for product release and a few necessary output signals for change release (if there is change to return), for example, C5 for a 5-cent change, C15 for a 15-cent change, etc.. No other signals are used. (a) Show the complete state transition table. (b) How many flip-flops are needed? Draw a block diagram of the circuit showing the key blocks, signals and wires. (c) How many K-maps are needed to derive the complete circuit? Why?