S.86 9-5

$m_v=25kg$
$\Delta T_1=18K$

$\Delta T_2=6K$

$c_v=4{,}1819\frac{kj}{\left(kg\cdot K\right)}=4181{,}9\ \frac{j}{\left(kg\cdot K\right)}$

$m_v=\frac{c_vm_v\Delta T_2}{c_v\Delta T_1}=\frac{4{,}1819\frac{kj}{\left(kg\cdot K\right)}\cdot25kg\cdot6K}{4{,}1819\frac{kj}{\left(kg\cdot K\right)}\cdot18K}=8{,}333...\approx8kg$