In an ideal heat engine, the absolute temperature of the heater is 5 times the absolute temperature of the refrigerator.

In an ideal heat engine, the absolute temperature of the heater is 5 times the absolute temperature of the refrigerator. If, without changing the heater temperature, increase the refrigerator temperature by 25%, then the efficiency of this engine will become equal …

Heat engine efficiency:
K = ((Tn – Tx) / Tn) * 100%,
where Tn is the heater temperature, Tx is the refrigerator temperature.
After changing temperatures, the following relations apply:
Tн = 5 * Tх;
Tх = 1.25 * Tх (Tх – the changed temperature of the refrigerator).
K = (5 * Tx – 1.25 * Tx) / 5 * Tx = (3.75 * Tx) / (5 * Tx) = 0.75 * 100% = 75%.
Answer: 75%.