a bag contains 4 red , 6 blue and 3 green balls . if 3 balls are picked at random , what is the probability that both are blue ?
"p ( both are red ) , = 6 c 3 / 13 c 3 = 10 / 143 c"
divide(choose(4, const_2.0), choose(add(add(4, 6), const_4.0), const_2.0))