Real gases rarely change one thing at a time. A balloon that climbs meets thinner air AND colder air together. The combined gas law takes it in one trip: $\dfrac{P_1 V_1}{T_1} = \dfrac{P_2 V_2}{T_2}$ — Boyle's ratio and Charles's ratio stacked. Cover the temperatures with a thumb and Boyle looks back at you; cover the pressures and there is Charles. It is not a third law to memorize — it is the two you know, holding hands.

Two working nuggets. First: inside a RATIO, matching units cancel — millilitres over millilitres is as honest as litres over litres, so mixed-unit problems often need no conversion at all. The one non-negotiable exception is temperature: kelvin, always, because only kelvin ratios mean anything. Second: solve algebraically FIRST, then substitute — six knobs is too many to juggle numerically, and the rearrangement is where the marks live.