Answer: The value of x is 30 dollars.
Explanation:
Since we have given that
Number of dollars in his investment account on Jan 1, 2001 = x
Number of dollars in his investment account on Jan 1, 2005 = 480
Number of years = 4
According to question, the amount of money in the account doubled each year.
so, it becomes,

Hence, the value of x is 30 dollars.