If a sample of helium gas has a volume of 200.0ml at .960 atm, what will the pressure be if the volume is reduced to 50.0ml (temperature and number of gas molecules remain constant)?

In order to calculate the pressure after the change in volume has occurred, we may apply Boyle's law, which states that the pressure of a fixed amount of gas at a constant temperature is inversely proportional to the volume of the gas. Therefore,
PV = constant
200 x 0.96 = 50 x P
P = (200 * 0.96) / 50
P = 3.84 atm