scholar:
'at Bbabylon' has a locative meaning in the sense of place or location and is similar to 'in Babylon'. However, most translations render the phrase as; 'for Babylon' which has a purposive meaning rather than a locative meaning. In either case, it shows that the Jews were exiled in Babylon and under Babylon's domination for the full period of 70 years.
So they were exactly 70 years or at least for 70 years in exile in Babylon?