$(4-$c$)$ Air at $1$MPa and $600C$ enters a converging nozzle with a velocity of $150$

$\frac{m}{s}.$ Determine the mass flow rate through the nozzle for a nozzle throat area

of $50c{m}^2$ when the back pressure is $($a$)0\mathrm{.}7$MPa and $($b$)0\mathrm{.}4$MPa.