what is the probability of getting only 1 head in a single throw of four fair coins ?
"one possible case is httt . p ( httt ) = 1 / 2 * 1 / 2 * 1 / 2 * 1 / 2 = 1 / 16 there are 4 c 1 = 4 possible cases . p ( 1 head ) = 4 * 1 / 16 = 1 / 4 the answer is c ."
divide(const_2, choose(add(const_3, const_3), const_3))