Water samples are taken from a river once a week. Let X denote the number of samples taken over a period of 20 weeks which are polluted. It is known that on average 10% of the samples are polluted....

Water samples are taken from a river once a week. Let X denote the number of samples taken over a period of 20 weeks which are polluted. It is known that on average 10% of the samples are polluted. Assuming independence of the outcomes of the sample analyses, what is the probability that X exceeds its mean by more than one standard deviation?