To calculate the final amount of money at the end of the period, considering that the interest is compounded continuously you have to use the following formula:
Where
A is the accrued amount at the end of the given time
P is the principal amount
r is the annual nomial interes expressed as a decimal value
t is the time period in years
For this investment, the initial value is P= $200
The interest rate is 6%, divide it by 6 to express it as a decimal value
The time is t= 20 years
After 20 years, the amount of money in the account will be $664.02