A 12 m 11 m 3.0 m basement had a high radon content. On the day the basement was sealed off from its surroundings so that no exchange of air could take place, the partial pressure of 222Rn was 1.2 10-6 mmHg. Calculate the number of 222Rn isotopes (t1/2 = 3.8 d) at the beginning and end of 27 days.