Physics Temperature and Kinetic Theory problem?

It's Daphne's birthday! Slash and Bucky have planned a big party to which Filbert the Feckless and hi evil minions Colonel Candle and Captain Flashlight are not invited. Bucky will fill balloons at a temperature of 20 degrees celsius with helium from a 75-liter tank of helium. Bucky finds that he can fill 300 balloons to a radius of 10 cm before he runs out of helium. uming that the pressure of the helium in each balloom is atmospheric, what was the initial pressure of the helium in the tank? How many helium atoms left the tank during the process.