Since the original cost is 2700, the fraction 1890 is of 2700 is 1890/2700. Since they both have a 0 at the end, we can cross that out to get 189/270. Next, since their digits add up to factors of 3 (1+8+9=18 and 2+7=9), we can factor 3 out to get 63/90. Repeating the process since these are factors of 3 too, we get 21/30. Doing it yet again since 21 and 30 are factors of 3, we get 7/10 as our answer