When the health department tested private wells in a county for two impurities commonly found in drinking water, it found that 20% of the wells had neither impurity, 40% had impurity A, and 50% had impurity B. (Obviously, some had both.) If a well is randomly chosen from those in the county, ﬁnd the probability distribution for Y, the number of impurities found in the well.

I know y = 0 is 20%.

But how is y = 1 70% and y = 2 10%?