Take a fixed amount of gas and move it from one state to another. Everything it can do is written in $\dfrac{P_1V_1}{T_1} = \dfrac{P_2V_2}{T_2}$ — P-one V-one over T-one equals P-two V-two over T-two. The subscript convention is fixed, and worth saying out loud once: 1 is the state before, 2 is the state after. $P$ is the ABSOLUTE pressure, $V$ is the volume, and $T$ is the ABSOLUTE temperature in kelvin.

Hold one of the three still and it cancels off both sides, leaving the law that carries the famous name. Hold the temperature: $P_1V_1 = P_2V_2$, Boyle's law — squeeze it and the volume pays. Hold the pressure: $\dfrac{V_1}{T_1} = \dfrac{V_2}{T_2}$, Charles's law — warm it and it swells. Hold the volume, as a sealed rigid cylinder does: $\dfrac{P_1}{T_1} = \dfrac{P_2}{T_2}$, Gay-Lussac's law — warm it and the pressure climbs with nowhere to go. So the exam question “which law is this?” is really the question “what is being held constant?”

Now the trap this lesson exists for. These are RATIOS, and a ratio only cancels its units when the scale's zero is the quantity's zero. Kelvin qualifies; Celsius does not, because its zero is a puddle freezing. Absolute pressure qualifies; gauge pressure does not, because its zero is a Tuesday afternoon in the shop. Put 20 °C and 40 °C into Charles's law and you will predict a doubling; the honest answer, 293 K to 313 K, is under seven per cent.

One sanity rail before every answer: decide which way the quantity should MOVE, then check that your number went that way. Compress a gas and its volume falls. Heat a sealed cylinder and its pressure rises. Half the wrong answers on this topic announce themselves by pointing the wrong direction.