Sinking Fund [2902]

You are doing a 10-year discounted cash flow analysis.  The HVAC, which must be replaced in four years, will cost $300,000.  The new HVAC will cost the same and will have to be replaced in 20 years.  How much must the owner save annually in the first four years of the analysis?  How much will he have to save in the last six years of the analysis?  Assume a reasonable return of 8%.