I'm looking for the effective annual compounded interest rate of an investment which requires me to save a fixed monthly amount, but the interest rate decreases as the investment gets closer to maturity. For example:
The investment starts out in a high growth phase of 10% for 24 months, which is followed by a medium growth of 8% for 12 months and the finally a low growth phase of 4% for 12 months. With each month requiring the same fixed payment.
I've been trying like crazy to find a formula, but most annuity type formulas work on changing payments over time rather than changing interest rates.
Any help would be greatly appreciated!