I don't know how a/b is somehow magically easier than b/a. They're both involving division. Most cars outside of the US report L/100km. For example, if you ha

I don't know how a/b is somehow magically easier than b/a. They're both involving division. Most cars outside of the US report L/100km. For example, if you have to go 250km and you're running at 8L/100km then you need 2.5x8 = 20L. If your tank holds 50L, then you'll need nearly a half tank. If you want to know how far you can get on a half tank, say 25L, then divide that by 8 to get roughly 6, or 600km.