(a) For a reversible engine, will you obtain a better efficiency by increasing the high-temperature reservoir by an amount T or decreasing the low-temperature reservoir

(a) For a reversible engine, will you obtain a better efficiency by increasing the high-temperature reservoir by an amount ΔT or decreasing the low-temperature reservoir by the same amount ΔT? Explain. 

(b) To illustrate your answer to this question, calculate the efficiencies of a reversible engine that initially uses reservoirs at 373 K and 923 K for ΔT = 50 K.