A first order reaction is 50% complete in 30 minutes at 300 K and in 10 minutes at 320 K. Calculate activation energy ($E_a$) for the reaction. [R = 8.314 J K$^{-1}$ mol$^{-1}$] [Given : log 2 = 0.3010, log 3 = 0.4771, log 4 = 0.6021]