a bag contains 3 red , 2 blue and 4 green balls . if 2 balls are picked at random , what is the probability that both are red ?
"p ( both are red ) , = 3 c 2 / 9 c 2 = 1 / 12 a"
divide(choose(3, 2), choose(add(add(3, 2), 4), 2))