c
(c)\(\frac{{{P^0} - {P_s}}}{{{P^0}}} = \frac{{\frac{w}{m}}}{{\frac{w}{m} + \frac{W}{M}}}\)

\(\because \,\frac{W}{M} > \frac{w}{m}\) \( \Rightarrow \frac{{640 - 600}}{{640}}\)

\( = \frac{w}{m} \times \frac{M}{W} \Rightarrow \frac{{40}}{{640}} = \frac{{2.175 \times 78}}{{m \times 39.08}}\) ;

\(m = \frac{{2.175 \times 78}}{{39.08}} \times \frac{{640}}{{40}}\)

\(m = 69.45\).