$\{\begin{array}{l}\begin{array}{rl}& s_1+s_2=120\\ & \frac{s_1}{80}+\frac{s_2}{40}=1\frac{3}{4}\end{array}\end{array}$

$\{\begin{array}{l}\begin{array}{rl}& s_1+s_2=120\\ & \frac{s_1}{80}+\frac{s_2}{40}=\frac{7}{4}|\cdot 80\end{array}\end{array}$

$\{\begin{array}{l}\begin{array}{rl}& s_1+s_2=120\\ & s_1+2s_2=140\end{array}\end{array}$

$\{\begin{array}{l}\begin{array}{rl}& s_1=120-s_2\\ & 120-s_2+2s_2=140\end{array}\end{array}$

$\{\begin{array}{l}\begin{array}{rl}& s_1=120-s_2\\ & s_2=20\end{array}\end{array}$

$\{\begin{array}{l}\begin{array}{rl}& s_1=100\\ & s_2=20\end{array}\end{array}$