Just plug in 500 for p, 0.2 for r (because 20% is equivalent to 0.20), and 2 for t:
\(I=p+prt\\I=(500)+(500)(0.2)(2)\\I=500+200\\I=700\)
So, if you start with $500 and charge an interest rate of 20% for 2 years, you will gain $200 to get $700 in total.
