Answer
$Ma= 0.897$
Work Step by Step
From the speed of sound relation $$
c=\sqrt{k R T}=\sqrt{(1.4)(0.287 \mathrm{~kJ} / \mathrm{kg} \cdot \mathrm{K})(-60+273 \mathrm{~K})\left(\frac{1000 \mathrm{~m}^2 / \mathrm{s}^2}{1 \mathrm{~kJ} / \mathrm{kg}}\right)}=293 \mathrm{~m} / \mathrm{s}
$$ Thus, the Mach number corresponding to the maximum cruising speed of the plane is
$$
\mathrm{Ma}=\frac{V_{\max }}{c}=\frac{(945 / 3.6) \mathrm{m} / \mathrm{s}}{293 \mathrm{~m} / \mathrm{s}}=\mathbf{0 . 8 9 7}
$$
