There are 1,000ml in a liter. There are 2 liters, so you originally had 2,000ml. You use 100, so you have 1,900ml left. Divide 1,900 by 400.
1900/400=4.75
We can round down, for a whole number.
So you can make 4 milkshakes with some milk left over for some pudding for me
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hope it helps