Perhaps the function is \( C(t) = \frac{20}{t+2} \)? Then \( C' = -20/(t+2)^2 = -1 \Rightarrow (t+2)^2 = 20 \Rightarrow t = \sqrt{20} - 2 \approx 4.47 - 2 = 2.47 \) — no.