there are 10 slate rocks , 15 pumice rocks , and 6 granite rocks randomly distributed in a certain field . if 2 rocks are chosen at random and without replacement , what is the probability that both rocks will be slate rocks ?
"10 / 31 * 9 / 30 = 3 / 31 the answer is b ."
multiply(divide(10, add(add(10, 15), 6)), divide(subtract(10, const_1), subtract(add(add(10, 15), 6), const_1)))