Đáp án:

 bài 12.

a,$5{m^3}$

b.$22500\left( N \right)$

bài 13

$1114\left( {kg/{m^3}} \right)$

Giải thích các bước giải:

 bài 12

$\begin{array}{l}
\left\{ \begin{array}{l}
{m_1} = 7,5kg\\
{V_1} = 5l\\
{m_2} = 7,5\tan  = 7500kg\\
{V_3} = 1,5{m^3}
\end{array} \right.\\
D = \frac{{{m_1}}}{{{V_1}}} = \frac{{{m_2}}}{{{V_2}}}\\
 \Rightarrow \frac{{7,5}}{5} = \frac{{7500}}{{{V_2}}} \Rightarrow {V_2} = 5000\left( l \right) = 5{m^3}\\
D = \frac{{{m_3}}}{{{V_3}}} = \frac{{{m_2}}}{{{V_2}}}\\
 \Rightarrow \frac{{{m_3}}}{{1,5}} = \frac{{7500}}{5} \Rightarrow {m_3} = 2250\left( {kg} \right)\\
{P_3} = 10{m_3} = 22500\left( N \right)
\end{array}$

bài 13

$\begin{array}{l}
\left\{ \begin{array}{l}
{m_1} = 80g = 0,08kg\\
{V_2} = 0,7l = 0,0007{m^3}\\
{m_2} = 0,7kg
\end{array} \right.\\
D = \frac{{{m_1} + {m_2}}}{{{V_2}}} = \frac{{0,08 + 0,7}}{{0,0007}} = 1114\left( {kg/{m^3}} \right)
\end{array}$