Do by hand to learn. See patterns 1 1 1 2 1 2 3 4 10 20 30 40 1 3 4 3 P(x)+P(x̄)=1 P(x|y)+P(x̄|y)=1 ... A,B disjoint 0 x y 0 x,y not 0 smoker - CO oximetry 49 57 24 370 drug job applicants 45 5 25 480 driving&texting 731 3054 156 4564 marijuana test 119 3 24 154 cancer 8 2 99 891 polygraph Rearranged! 42 9 15 32 INDEPENDENCE: rows and columns are multiples of each other 10 30 20 60 3blue1brown 9 1 89 901