The efficiency of a Carnot engine which operates between the two temperatures $T_{1} = 500 \ K$ and $T_{2} = 300 \ K$ is: (in $\%$)

$A$ Carnot engine has an efficiency of $40\%$. The temperature of its sink is $300 \ K$. To increase the efficiency by $50\%$ of its original efficiency while keeping the sink temperature constant,by how many $K$ must the source temperature be increased?

$A$ Carnot engine with its cold body at $17\,^{\circ}C$ has $50\%$ efficiency. If the temperature of its hot body is now increased by $145\,^{\circ}C$,the efficiency becomes...... $\%$

Two heat engines $X$ and $Y$ of same efficiency are connected in series in such a way that the sink of $X$ works as the source of $Y$. $X$ receives heat at $900 \,K$ and rejects some heat to its sink at $T \,K$, and in turn, $Y$ rejects heat to its sink at $400 \,K$. Then the temperature $T$ is: (in $\,K$)

For which of the following options will the efficiency of a Carnot engine be the highest?

$A$ Carnot engine whose low-temperature reservoir is at $7\,^{\circ}C$ has an efficiency of $50\%$. It is desired to increase the efficiency to $70\%$. By how many degrees should the temperature of the high-temperature reservoir be increased in $K$?