70nm at original ground speed (180kts) would take 23.3 minutes.  

Aircraft has covered 70nm in 20.3 minutes = ground speed 207kts.  

Remaining distance to run is 320 – 70 = 250nm and at revised ground speed of 207kts this will take 72.5 minutes.  

Total elapsed time = 20.3 + 72.5 = 92.8 minutes (1h33m).  

Departure time is 1200 add elapsed time 1h33m, ETA = 1333.