Approximately 1/4 of multiple choice questions involve ratios. The reason for this is that calculators aren't allowed on paper 1 so instead of calculating  final values answers are give in ratios. I use this trick a lot in studyIB, the student revision site. Student's often find these sort of questions difficult so I thought I'd make a short video explaining how to go about them. If there is only one variable they can be solved quickly using a simple proportional relationship but I have shown the full working to be on the safe side.