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Amount of carbon present in graphite = 1.2 mg = 0.0012 g

No. of carbon atoms present in 12g of carbon = 6.023*10^23

Therefore, No. of carbon atoms present in 1g of carbon = 6.023*10^23/12

Therefore, No. of carbon atoms present in 0.0012g of carbon = 6.023*10^23/12 * 0.0012 = 6.02 * 10^19 atoms (Answer)