Entropy is a measure of molecular disorder or randomness in any system. Useful work is obtained when there is a ordered molecular motion, so entropy of a system is the measure of thermal energy per unit temperature which is unavailable for doing any useful work. Entropy change across a normal shock is given by

$$\frac{{{p_{02}}}}{{{p_{01}}}} = {e^{ – \left( {{s_2} – {s_1}} \right)/R}}$$

$$\Rightarrow \frac{{{p_{02}}}}{{{p_{01}}}} = {e^{ – \left( {220} \right)/287}} = 0.46$$

Therefore, from normal shock properties table, for, $\frac{{{p_{02}}}}{{{p_{01}}}} = 0.46$, upstream Mach number $M_1$, is around $2.6$.