Question

A container with a volume of `5` `L` contains a gas with a temperature of `360` `K`. If the temperature of the gas changes to `120` `K` without any change in pressure, what must the container's new volume be?

Answer 1

Start with the 'combined gas law': `(P_1V_1)/T_1=(P_2V_2)/T_2`. Pressure is constant, so we can ignore it and be left with `(V_1)/T_1=(V_2)/T_2`.
Rearranging, `V_2=(V_1T_2)/T_1=(5*120)/360=5/3` `L=1.67` `L`.

Explanation:

There are a number of different gas laws that link the pressure, temperature and volume of gases.

You could memorise Boyle's Law, Charles' Law and Gay Lussac's Law. I find it simpler, though, to just learn the Combined Gas Law that combines all of them, and then eliminate the things I don't need.

More understanding, less memory: my approach to learning physics. ;-)