a)

$\displaystyle \Delta t = 2 + 5 + 5 = 12 \text{ phút} = 720 \text{ s}$

$\displaystyle Q = P \cdot \Delta t = 800 \cdot 720 = 576000 \text{ J} = 576 \text{ kJ} \Rightarrow \textbf{Đúng}$

b)

$\displaystyle \begin{cases} P \cdot 2 = m_0 c (50 - 20) \\ P \cdot 5 = (m_0 + m_t) c (100 - 70) \end{cases}$

$\displaystyle \Rightarrow \frac{2}{5} = \frac{30 m_0}{30(m_0 + m_t)} \Rightarrow 2m_0 + 2m_t = 5m_0 \Rightarrow m_t = 1,5 m_0 \Rightarrow \textbf{Sai}$

c)

$\displaystyle P \cdot 5 = m_0 c (70 - 50) + m_t c (70 - t_x)$

$\displaystyle \text{Từ (b) có } P = 15 m_0 c \text{ và } m_t = 1,5 m_0:$

$\displaystyle 5(15 m_0 c) = 20 m_0 c + 1,5 m_0 c (70 - t_x)$

$\displaystyle \Rightarrow 75 = 20 + 1,5(70 - t_x) \Rightarrow t_x = \frac{100}{3} \approx 33,3^\circ\text{C} \Rightarrow \textbf{Sai}$

d)

$\displaystyle P \cdot \Delta t_1 = m_0 c \Delta T_1$

$\displaystyle \Rightarrow P \cdot (2 \cdot 60) = 1 \cdot 4200 \cdot (50 - 20)$

$\displaystyle \Rightarrow 120 P = 126000 \Rightarrow P = 1050 \text{ W} \Rightarrow \textbf{Sai}$