Answer
$T_4=−9.00^{∘}C$
Work Step by Step
Assuming the turbine to be isentropic, the air temperature at the turbine exit is determined from $$
T_4=T_3\left(\frac{P_4}{P_3}\right)^{(\mathrm{k}-1) / \mathrm{k}}=(343 \mathrm{~K})\left(\frac{100 \mathrm{kPa}}{250 \mathrm{kPa}}\right)^{0.4 / 1.4}=264 \mathrm{~K}=-\mathbf{9 . 0} ^{\circ} \mathrm{C}
$$
