A fixed amount of gas has three knobs: pressure $P$, how hard it drums on its walls; volume $V$, the space it is allowed to fill; and temperature $T$, the average kinetic energy of its molecules. The three classical laws each pin one knob and watch the other two. Everywhere in this chapter, subscript 1 is the state BEFORE and subscript 2 is the state AFTER — $P_1$ is the pressure you started with, $P_2$ the pressure you ended with, and so on for every letter.

$P_1 V_1 = P_2 V_2$ is Boyle's law — read aloud P-one V-one equals P-two V-two — at constant temperature. $\dfrac{V_1}{T_1} = \dfrac{V_2}{T_2}$ is Charles's law, V-one over T-one equals V-two over T-two, at constant pressure. $\dfrac{P_1}{T_1} = \dfrac{P_2}{T_2}$ is Gay-Lussac's law, at constant volume. Whichever of the four letters the question leaves blank is the one you solve for.

One rule outranks all three, and it is where most marks are lost: every $T$ here is an absolute temperature, in kelvin. $T(\mathrm{K}) = t(^\circ\mathrm{C}) + 273$ — 273.15 if the paper wants it, 273 on this one. Ratios need a scale that starts at true zero, and Celsius does not: 20 °C is emphatically not twice as hot as 10 °C, but 293 K really is 293 times as hot as 1 K. Convert first, always.