A black body at high temperature T K radiates energy at the rate of EW/m2. When the temperature falls to (T2)K, the radiated energy will be:
E4
E2
2E
E16
Q ∝ T4
⇒ Q1Q2 = T14T24 ⇒ EE' = T4(T2)4
which gives E' = E16