A heat engine is involved with exchange of heat of 1915J, – 40J, +...

${I}^{st}$ case when ${Q}_{4}$ is negative $η = \dfrac{W}{Σ{Q}_{+}} = \dfrac{{Q}_{1} + {Q}_{2} + {Q}_{3} + {Q}_{4}}{{Q}_{1} + {Q}_{3}} = 0 . 5$
$⇒ \dfrac{1915 - 40 + 125 + {Q}_{4}}{1915 + 125} = 0 . 5$
$⇒ 1915 - 40 + 125 + {Q}_{4} = 1020$
$⇒ {Q}_{4} = 1020 - 2000 ⇒ {Q}_{4} = - Q = - 980J$

$Q = 980J$

${II}^{nd}$ case when ${Q}_{4}$ $\dfrac{W}{Σ{Q}_{+}} = \dfrac{{Q}_{1} + {Q}_{2} + {Q}_{3} + {Q}_{4}}{{Q}_{1} + {Q}_{3} + {Q}_{4}} = 0 . 5$ is positive
${Q}_{4} = - 1960$