The compound interest formula is A=P(1+r100)t A = P \left( 1 + \frac{r}{100} \right)^t A=P(1+100r)t. Using the amounts for the third and fourth years, we can set up the equation 73,20566,550=(1+r100). \frac{73,205}{66,550} = \left( 1 + \frac{r}{100} \right). 66,55073,205=(1+100r). Solving this gives r=10% r = 10\% r=10%.