To calculate problems abound about compounding interest use the equation A = P (1 + r/n)^(nt), where A is the future price, P is the principal amount, r is the interest rate, n is the number of times the interest is compounded per year and t for the total years. To solve, A = 100 (1 + 0.08/1)^(1 x 15) = 317.22.