F.3 maths: Percentage?

Let n -- no. of yrs. the machine had been bought in 2004,
Let P -- original value of the machine,
Then, value of the machine in 2004 = P/2

P/2 = P(1 - 16%)ⁿ　　　《公式 : A = P(1+r%)ⁿ》
1/2 = (1 - 0.16)ⁿ
(0.84)ⁿ = 1/2
n log 0.84 = log 0.5
∴ n = (log 0.5) / (log 0.84) = 3.98 (to 2 dec. places) ≒ 4 yrs

Thus, the factory bought the machine in 2000.