The probability that a certain radioactive mass emits no particles in a one-minute time period is 0.1353. What is the mean number of particles emitted per minute? From the above I was able to deduce that P(X>=1) is 1 - 0.1353 = 0.8647. 0.8647 chance of at least 1 particle being emitted in a minute. I'm not sure how to calculate the mean number of particles per minute though.