${\mathrm{I}}^{\mathrm{st}}$ case when $Q_4$ is negative $\eta =\frac{W}{\Sigma Q_{+}}=\frac{Q_1+Q_2+Q_3+Q_4}{Q_1+Q_3}=0.5$
$\Rightarrow \frac{1915-40+125+Q_4}{1915+125}=0.5$
$\Rightarrow 1915-40+125+{\mathrm{Q}}_4=1020$
$\Rightarrow Q_4=1020-2000\Rightarrow Q_4=-Q=-980\mathrm{J}$

$Q=980\mathrm{J}$

${\mathrm{II}}^{\mathrm{nd}}$ case when $Q_4$ $\frac{\mathrm{W}}{\Sigma {\mathrm{Q}}_{+}}=\frac{Q_1+Q_2+Q_3+Q_4}{Q_1+{\mathrm{Q}}_3+Q_4}=0.5$ is positive
$Q_4=-1960$