Solve the below problem:
Q: Dixon Shuttleworth has a large sum of money that he wants to invest to finance his retirement. He has been presented with three options. The first investment offers a 5% return for the first five years, a 10% return for the next five years, and a 20% return thereafter. The second investment offers 10% for the first ten years and 15% thereafter. The third investment offers a constant 12% rate of return. Determine which of these investments is the best for Dixon if he plans to retire in the following number of years.