For an air$-$breathing jet engine, specific thrust is defined as thrust divided by air mass flow rate.

Consider an engine that is flying at a velocity equal to $250$ meters per second. For every

kilogram $\frac{?}{?}$ second of air mass flow, $0\mathrm{.}040kg\frac{m}{sec}$ mass flow of fuel is injected into the engine. The

exit pressure and the entrance pressure both equal the ambient pressure, which is $0\mathrm{.}7$ atm $.$ What

must be the value of the exhaust velocity $u_e$ if a specific thrust equal to $400\frac{m}{s}$ is desired?