a bag contains 4 red , 4 blue and 2 green balls . if 2 ballsare picked at random , what is the probability that both are red ?
"p ( both are red ) , = 4 c 210 c 2 = 4 c 210 c 2 = 6 / 45 = 2 / 15 a"
divide(choose(4, 2), choose(add(add(4, 4), 2), 2))