Finance question regarding an investment?

Question

An investment receives 8.7% per year. You have a current balance of $9,000 (includes accumulated interest) in the investment account. How long ago would you have needed to invest 2/3 of the current value?

Answer 1

P = A(1 + i)^n

6000 is 2/3 of 9000

9000 = 6000(1.087)^n

(1.087)^n = 1.5

n(ln (1.087)) = ln (1.5)

n = (ln (1.5))/(ln 1.087)

n = 4.86 yrs
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