How much money invested at 5% compounded continuously for 3 years will yield $820?

Percentage of interest at which the money is invested = 5%
Time for which the money is invested = 3 years
The final amount after 3 years = $820
Let us assume the principal amount = x dollars
Then
820 = x (1 + 0.5)^3
820 = x(1.05)^3
x = 820/(1.05)^3
   = 708.35 dollars
From the above deduction, we can conclude that the principal amount invested was $708.35.