The value of power dissipated across the zener diode V z = 15   V connected in the circuit as shown in…

Voltage across $R_S=22-15=7 \mathrm{V}$

Current through $R_{\mathit{S}}=I=\frac{7}{35}=\frac{1}{5} \mathrm{A}$

Current through  $90\mathrm{\Omega }=I_2=\frac{15}{90}=\frac{1}{6} \mathrm{A}$

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

Power through zener diode

$P=VI$

$P=15\times \frac{1}{30}=0.5$ watt

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