Problem: a certain junior class has 1,000 students and a certain senior class has 800 students . among these students , there are 20 siblings pairs , each consisting of 1 junior and 1 senior . if 1 student is to be selected at random from each class , what is the probability that the 2 students selected at will be a sibling pair ?
Rationale: "total number of ways of choosing one student from each group is = 800 * 1000 number of cases in which a sibling pair will be got is = 60 thus the probability that the 2 students selected will be a sibling pair is = 20 / ( 800 * 1000 ) = 1 / 40,000 d"
annotated_formula: divide(1, const_3)