This problem requires us to calculate, the value of investment after 10 and 25 years, and also tell the time after which intial investment amount will double. Investment rate and initial investment amount is given in the question.
Value of investment after 10 year = 600(1+8%)^10 = $ 1,295
Value of investment after 25 year = 600(1+8%)^25 = $ 4,109