How much did Charles originally invest?

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Charles invested in an account on April 1 at a variable rate of interest. Interest on the investment will be paid on June 30. The interest rate began at %5.5 simple interest, increased to %7 effective April 17, then fell by %0.5 effective May 31. The interest credited to the account on June 30 was $80.31. How much did Charles originally invest?

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