Question: how many copies did back in black sell
[{'answer': 'an estimated 50 million', 'provenance': [{'wikipedia_id': '196945', 'title': 'Back in Black', 'start_paragraph_id': 4, 'start_character': 103, 'end_paragraph_id': 4, 'end_character': 126, 'bleu_score': 1.0, 'section': 'Section::::Abstract.'}]}, {'provenance': [{'wikipedia_id': '196945', 'title': 'Back in Black', 'start_paragraph_id': 4, 'start_character': 0, 'end_paragraph_id': 4, 'end_character': 602, 'bleu_score': 0.8987097658633421, 'meta': {'yes_no_answer': 'NONE', 'annotation_id': 8905115350513045530}, 'section': 'Section::::Abstract.'}]}]