Answer :
Given a family consumes 30 l 750 ml of vegetable oil and cooking oil in a month .
We know 1 litre = 1000 ml , So
Total oil consumed = 30 ( 1000 ) + 750 = 30000 + 750 = 30750 ml
And
the consumption of vegetable oil is 12 l 255 ml , So
Total vegetable oil consumed = 12 ( 1000 ) + 255 = 12000 + 255 = 12255 ml
So,
Total cooking oil consumed = 30750 - 12255 = 18495 ml = 18 l 495 ml ( Ans )