This is the boss. No calculator — the numbers are chosen to fit in your head: $V_m = 22.4\ \mathrm{L/mol}$ at STP, and every ratio in the climb is a clean fraction. One balloon, launch to altitude, and every answer feeds the next: count the moles at the fill, then ride the combined law up — pressure's stretch first, the chill's cut second. Write each line down as you go; one early slip rises all the way to reporting altitude with you.

Everything from the chapter rides in it. 5★ under par here is the chapter crown. Pressure test — begin.