Answer to Question 4

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The maximum current must be 10 mA.  The current flowing through the series resistor is the same as the current in the diode.

The voltage drop across the resistor is 12 V - 5.6 V = 6.4 V

Resistance = 6.4 V 10 10-3 A = 640 W

The nearest E24 series resistor is 620 W, but that would allow too big a current to flow (10.3 mA).  The next one up is 680 W