Think of the question as 2mm: 350mm then just convert the 350mm to cm by dividing by 10. 350mm=35cm.
Now you’ve got 2mm:35cm. All you have to do now is divide the ratio to get from 2mm to 1mm and so you just divide the whole ratio by 2 and end up with 1mm:17.5cm. I think that’s what the question is asking. Hope this is helpful.
