Agas has a volume of 5.0 l at a pressure of 50 kpa. what happens to the volume when the pressure is increased to 125?

The volume becomes two. You have to use the equation P1 x V1 = P2 x V2 
P is pressure and V is volume.
P1 = 50     P2 = 125
V1 = 5       V2 = v (we don't know what it is)
Then set up the equation:
50 times 5 = 125 times v
250 = 125v
the divide both sides by 125 and isolate v
2 = v
Therefore the volume is decreased to 2.
Also, Boyle's Law explains this too: Volume and pressure are inversely related, This means that when one goes up the other goes down (ie when pressure increases volume decreases and vice versa). Becuase the pressure went up from 50 KPa tp 125 KPa the volume had to decrease.