Question

Sung Lee invests $4,000 at age 18. He hopes the investment will be worth $16,000 when he turns 25. If the interest compounds continuously, approximately what rate of growth will he need to achieve his goal? Round to the nearest tenth of a percent.

Answer 1

The growth rate he needs to achieve his goal is approximatelly 19.8%

Explanation:

Since the sum will be compounded continuously we have to use the appropriate formula given below:

M = C*e^(r*t)

Where "M" is the final amount, C is the initial amount, r is the interest rate and t is the time elapsed. Since Sung Lee will invest that sum at 18 years old and he wants to recieve the return at 25, then the time elapsed is given by 25 -18 = 7 years. We can now apply the data to the formula:

16000 = 4000*e^(r*7)

4000*e^(7*r) = 16000

e^(7*r) = 16000/4000 = 4

ln[e^(7*r)] = ln(4)

7*r = ln(4)

r = ln(4)/7 = 0.198

The rate of interest is given by (r)*100%, so we have (0.198)*100% = 19.8%.