In a Carnot cycle, heat absorbed at hot reservoir is 1000 J. Efficiency 25%. Heat rejected at cold reservoir:

Work done = η × Q_h = 0.25 × 1000 = 250 J. By 1st law (cyclic): Q_h = W + Q_c We have → Q_c = Q_h - W. That is → Q_c = 1000 - 250. So → Q_c = 750 J.