In testing the water supply for various cities for two kinds of impurities commonly found in water, it was found that 40% had an impurity of type A and 50% had an impurity of type B. 20% of the water supplies had neither sort of impurity (that is 20% did not have type A impurity and it did not have type B impurity). If a city is chosen at random, what is the probability that its water supply has exactly one type of impurity?