For this case we have to, by definition:
1 Kilogram is equivalent to 1000 grams
If Ali bought 3 kg of mandarin orange, we make a rule of three to determine how many grams are:
1 kg ------------> 1000 g
3 kg ------------> x
Where "x" represents the amount of grams that Ali bought.

If Mia bought 3250 grams of mandarin oranges, and we want to know how many grams I buy more than Ali, we subtract:

So, Mia bought 250 grams more mandarin orange than Ali.
Answer:
250 grams