Article in English: For many pennies, one tablespoon (15 g) of salt in 1/2 cup (4 oz) of vinegar will get the job done. Stir the mixture to dissolve the salt. If you don't have vinegar, use lemon or even orange juice. Copper oxide (the gunk on your pennies) dissolves in weak acid, and that's just what all three of these liquids are.

Summary in Vietnamese:
Khuấy 1 thìa cà phê (5 gr) muối vào 1/4 cốc (khoảng 60 ml) giấm.