How does air pressure in a rocket work

The launch of a compressed air rocket proceeds in three stages; the pressurization of the body tube, the initial acceleration along the launch tube, and the expulsion of the compressed air from the rear of the rocket. Here is a computer animation of the launch of a compressed air rocket

Here is a site if you need more help.  https://spaceflightsystems.grc.nasa.gov/education/rocket/rktstomp.html

if you add everything up correctly you should get 69 i got this because 50 plus 19 is it