Consider an automobile tire which is inflated indoors in the winter time. The tire is then moved outside, where the temperature is lower. Since the tire is sealed, the number of moles does not change. We can assume that, to very good approximation, the volume of gas in the tire also does not change. As the tire cools, the pressure will change.
The ideal gas law will be satisfied by the gas in the tire during this process. On the screen the ideal gas law has been rearranged to show that, for this example, pressure is a constant number times temperature as the tire is cooled. Therefore, as mT decreases, mP must also decrease. The next screen will show you how to solve for the pressure decrease if you know the temperature change.