Interest = Principal * n (years)* rate/100 so based on the first problem of $780 = ($3000 principal *2 years * r) /100, r=13. So in this case the interest rate was 13%.
We can use the 13% to calculate the second part of the question by using the formula of i=Pnr/100 to get:
i=4500*9*13
So i = $5265
