After 3/5 moles has escaped, what has remianed is 2/5 moles. Now Initially the same volume was occupied by 1 mole of gas at 300 K is occupied now by 2/5 moles of air at T.
So, Initially, 1 . V = 300 .R .1
Finally, 1 . V = T . R . 2/5
Solving T = 750 K = 477 ° C