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A::B::DSolution :

E=Energy radiated per unit area per unit time <br> Rate of heat flow =Energy radiated <br>(a) Energy per unit time` =ExxA` <br> `So, E_(Al)=(eOT^4xxA)/(eOT^4xxA')` <br> `=(4 pi r^2)/(4pi (2r)^2)=1:4` <br> `(b) Emissivity of both are same` <br> ` (M_1s_2dT_1)/(M_1s_2dT_2)=1` <br> `implies (dT_1)/(dT_2)=(M_2s_2)/(M_1s_1)` <br> =(rho_(2)4 pi r_(2)^(3)xxs_2)/(rho_(1)4 pi r_(1)^(3)xxs_(1))` <br> `(1xx3.4xx8 pi xx390)/(4xx1xxpixx90)` <br> `2.9:1`.