Problem: when a random experiment is conducted , the probability that event a occurs is 1 / 4 . if the random experiment is conducted 4 independent times , what is the probability that event a occurs exactly twice ?
Rationale: "one case is : 1 / 4 * 1 / 4 * 3 / 4 * 3 / 4 = 9 / 256 the total number of possible cases is 4 c 2 = 6 p ( event a occurs exactly twice ) = 6 * ( 9 / 256 ) = 27 / 128 the answer is b ."
annotated_formula: subtract(1, divide(const_2, 4))