In desalting groundwaters that contain a significant amount of sulfates, the concentrate that is generated during the desalting process can sometimes be treated with lime to recover gypsum and other salts. Because the high pH process is tough on equipment, the equipment’s useful life is uncertain.

A treatment train with an initial cost of

$90,000 has an operating cost of $20,000 per month. The revenue from the sale of calcium sulfate is $22,000 per month. Determine how many months the equipment must last to recover the investment at i  0.5% per month using (a)

hand solution,

(b) a calculator, and

(c) a single-cell spreadsheet function.