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The abacus is an ancient computation device, in skilful hands, it can calculate sums as quickly as most modern gadgets.
Possible Corrections:
The abacus is an ancient computation device, but in skilful hands, it can calculate sums as quickly as most modern gadgets. (comma and conjunction)
The abacus is an ancient computation device; however, in skilful hands, it can calculate sums as quickly as most modern gadgets. (semicolon to join two dependent clauses)
Although the abacus is an ancient computation device, in skilful hands it can calculate sums as quickly as most modern gadgets. (Turn one of the independent clauses into a dependent clause)
The abacus is a modern computation device. In skilful hands it can calculate sums as quickly as most modern gadgets. (2 separate sentences)