A zener diode of power rating 1 . 6W is to be used as voltage...

Power rating of zener diode $= 1 . 6W$

From the diagram above,

${V}_{A} - {V}_{B} = 8V$

Power is given by

$P = Vi ⇒ i = \dfrac{1 . 6W}{8V} = 0 . 2A$

Also, ${V}_{C} - {V}_{A} = 10V - 8V = 2V$

Thus, using Ohm's law,

${R}_{s} = \dfrac{2V}{0 . 2A} = 10Ω$