The Combined Gas Law: You Set Five Things, the Sixth Belongs to the Physics

Two cylinders holding the same amount of the same gas. You set the pressure, volume and temperature before, and the pressure and temperature after; the volume after is worked out from P1V1/T1 = P2V2/T2 and is never a slider. Both cylinders stand on one shared volume scale, so a gas that shrinks always looks smaller. The two PV/T cards agree because V2 was computed to make them agree — that is the law being displayed, not evidence for it. The demonstration that does carry weight is Show both routes: squeeze first then heat, or heat first then squeeze, and the middle volumes come out different while the destination does not.

Route A: Boyle first1.00 L then 1.50 L
Route B: Charles first3.00 L then 1.50 L
Pressure ratio0.5000
Temperature ratio1.500
T1 in °C26.9 °C
T2 in °C176.9 °C

Which law is thisPressure and temperature both change, so only the combined gas law applies.

Temperatures are in kelvin and pressures are absolute, never gauge. The amount of gas is held, which is why both cylinders carry the same number of dots; a leak, a gas dissolving into a liquid or a chemical reaction all break the law. Both states are equilibrium states, so nothing here is a claim about the journey between them.
Volume after  V2 = P1V1T2 / (T1P2)
1.50 L
Volume factor  V2/V1 = (P1/P2)(T2/T1)
0.7500
PV/T before, kPa L / K  P1V1 / T1
0.6667
PV/T after, kPa L / K  P2V2 / T2
0.6667
Pressure before · P1100.0 kPa
Volume before · V12.00 L
Temperature before · T1300.0 K
Pressure after · P2200.0 kPa
Temperature after · T2450.0 K
Volumes print to 2 dp, pressures and temperatures to 1 dp, PV/T to four significant figures, and every box is rounded on its own. Do not check the law by multiplying the numbers on screen — it fails to come out in about four states in five, and the arithmetic is fine; it is the second decimal place.