Magnitude of thrust of rocket (q50)

[jack1234]

For this question:
http://tinyurl.com/ynkshx

I just use ma=(3000-800)*20 =44kN to get the answer, and the answer is correct.
and haven't make use of
thrust=exhause velocity * dm/dt
or
change in velocity=exhause velocity*ln|initial mass/final mass|

So, just want to confirm am I solved this problem with correct approach, if not, what is the correct solution?

 

[Dick]

That's right. It's not really much of a rocket problem, is it?