Ta có $\cos\left(\dfrac{\pi t}{12} + \dfrac{\pi}{3}\right) \leq 1$$\\$ $\Rightarrow h \leq 3.1 + 10 = 13$$\\$ Mực nước đạt độ cao lớn nhất bằng $13 \ (cm)$ khi:$\\$ $\cos\left(\dfrac{\pi t}{12} + \dfrac{\pi}{3}\right) = 1$$\\$ $\Rightarrow \dfrac{\pi t}{12} + \dfrac{\pi}{3} = k2\pi$ $(k \in \mathbb{Z})$$\\$ $\Rightarrow \dfrac{t}{12} + \dfrac{1}{3} = 2k$$\\$ $\Rightarrow \dfrac{t}{12} = -\dfrac{1}{3} + 2k$$\\$ $\Rightarrow t = -4 + 24k$$\\$ Vì $0 \leq t \leq 24$:$\\$ $\Rightarrow 0 \leq -4 + 24k \leq 24$$\\$ $\Rightarrow 4 \leq 24k \leq 28$$\\$ $\Rightarrow \dfrac{1}{6} \leq k \leq \dfrac{7}{6}$$\\$ Vì $k \in \mathbb{Z} \Rightarrow k = 1$$\\$ $\Rightarrow t = -4 + 24.1 = 20$