B$)$An insulating powder is densely packed in the annular space between twoconcentric spheres with radi $75$ mm and $50$ mm$,$ The inner spherce isunifornly heated with electric power input of $30$ W$.$ Steady state temperatureattaincd by inner sphere is $120\backslash$deg C and that by outer sphere is $30\backslash$deg C$.$ Negiectthermal resistance offered by the material of the splheres.a$)$ Calculate thermal conductivity of the powder$\mathrm{.}0-$b$)$ Calculate the thermal resistance offered by insulating powder $30$