multiply 3,000 by 0.03 which will get you 90 which is 3% of that for one year and for two years you just multiply 90 by 2 and get 180. subtract 180 from 3,000
It could sometimes be proportional. The reasoning here is that if all the items have different prices it won't be, however if the items you are ordering all have the same price they will be!
