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For inches to cm in my head, I multiply by 10 and then divide by 2 twice. E.g. 7 inches -> 70 -> 35 -> ~17.5 cm.



I do something similar but, because I'm faster at multiplying, I do "its double plus its half". E.g. 7 inches = 14 + 3.5 = 17.5 cm




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