a bag contains 3 red , 4 blue and 4 green balls . if 2 ballsare picked at random , what is the probability that both are red ?
"p ( both are red ) , = 3 c 211 c 2 = 3 c 211 c 2 = 3 / 55 c"
divide(choose(3, 2), choose(add(add(3, 4), 4), 2))