1-"Money" has to be invested in the bank to get a profit at the rate of 20% annually this "Money " is called investment and the profit is called interest and the value that you need to get is amount (800$)
2- according to your question you need to get 800$ after 21 months at the rate of interest (20% per annum)
3-The formula used here is
Now which point you dont get? or is it the fourth option
4)All of these