Here is a physics HW question that i don't know how to do~?

here's the question: a flack of volume 300ml contains air at a pressure of 500000 pa, and a temperature of 27c. if the flask loses ,molecules at rate of 30000000000000000000 per second,after how much time will the pressure in the flask be reduced to half of its original value?(ume the temperature of the air is constant during this time)