Pressure is constant means Charles' law can be applied.
Let, final temperature is TK (T2), initial temperature is 300 K(T1); initial volume is V(V1) and final volume isV3 (V2) Then from Charles' law we have,V1T1=V2T2 or, V300=V3×1T or, T=3003=100 K∴Final temperature is 100 K.