Answer
(a) stay the same
(b) decrease
Work Step by Step
(a) We know that the efficiency of a Carnot engine is given as $e=1-\frac{T_c}{T_h}$. This equation shows that if both $T_c$ and $T_h$ are doubled then the efficiency will remain the same because the ratio$\frac{T_c}{T_h}$ remains the same.
(b) We know that if $T_c$ and $T_h$ are increased by $50K$, then the ratio $\frac{T_c}{T_h}$ approaches to 1 and thus the efficiency will decrease.
