The answer is: (4X1000)-(12X330)= 40, ao 40 ml more. You may wonder why 1 litre is 1000, as 1 litre of cola= 1000 ml of cola so we have to convert 1 L to 1000ml in the anwer. Rob buys more cola than Jodie means they are asking the differences of their cola volume, therefore, it is the cola that Rob has minus the cola that Jodie has.