a bag contains 4 red , 5 blue and 3 green balls . if 3 ballsare picked at random , what is the probability that both are red ?
"p ( both are red ) , = 4 c 3 / 12 c 3 = 1 / 55 e"
divide(choose(4, 3), choose(add(add(4, 5), 3), 3))