First you will convert 33 months into years which is 2 years and 10 months.
You will then add the 2 years and the 11 years together which gets you 13 so now you have a total of 13 years and 10 months. (this information will be used later)
Now we will start by multiplying the 20000 by 16% and we should get a total of 1200. That means the interest is 1200 per year so lets first take our 13 years and multiply it by the 1200 (13x1200). You should have 15600 as an answer (save this number for later) since we have 10 months were going to take that 1200 and divide it into 12 month meaning the monthly interest should be 100 per month since we have 10 months we will multiply 100 by 10 and get 1000. Now lets bring back that 15600 and add the additional 1000 to it, our answer should be 16,600, and then we add the original 20000 to the 16600 and your final answer should be 36600.
