The value of power dissipated across the zener diode ( V _ z = 15V)...

Voltage across ${R}_{S} = 22 - 15 = 7V$

Current through ${R}_{S} = I = \dfrac{7}{35} = \dfrac{1}{5}A$

Current through $90Ω = {I}_{2} = \dfrac{15}{90} = \dfrac{1}{6}A$

Current through zener $= \dfrac{1}{5} - \dfrac{1}{6} = \dfrac{1}{30}A$

Power through zener diode

$P = VI$

$P = 15 \times \dfrac{1}{30} = 0 . 5$ watt

$P = 5 \times {10}^{- 1}$ watt