A container with a volume of #14 L# contains a gas with a temperature of #160^o K#. If the temperature of the gas changes to #80 ^o K# without any change in pressure, what must the container's new volume be?
Assuming the gas is ideal, this can be calculated in a few different ways. The Combined Gas Law is more appropriate than the Ideal Gas Law, and more general (so being familiar with it will benefit you in future problems more frequently) than Charles' Law, so I'll use it.
Rearrange to make proportional variables obvious
Pressure is constant, so whatever it is, it divided by itself will be
End with the same units you started with
This answer makes intuitive sense. If the pressure is constant, decreasing the temperature should decrease the volume, since less energetic particles will take up a smaller amount of room.
It worked here because this equation was based on how all the same variables varied with respect to each other, and I started with volume in a non-standard unit and ended with volume a non-standard unit.