You know, I presume, that 1 kg is much larger than 1 g. So 1250 mg will be a much smaller number of kg.
A more detailed way of looking at it: There are 1000 "milli"gram in a gram: and 1000 grams in a "kilo"gram.
So - the "grams" cancel.
Since you have "mg" and want to end with kg, you need to multiply by kg/mg: mg(kg/mg)= kg
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