A sum of money invested at simple interest amounts to 2480 at the end of four years and 4080 at the end of eight years. Find the principal.

The simple interest earned over the additional four years (from year four to year eight) is the difference between the final amounts: 4080 - 2480 = 1600. The interest earned per year is 1600 / 4 = 400, making the total interest for the first four years equal to 1600. Using the simple interest concept, the principal is the amount at the end of four years minus the interest earned over those four years: 2480 - 1600 = 880.

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