a bag contains 6 red , 5 blue and 2 green balls . if 2 ballsare picked at random , what is the probability that both are red ?
"p ( both are red ) , = 6 c 213 c 2 = 6 c 213 c 2 = 5 / 26 c"
divide(choose(6, 2), choose(add(add(6, 5), 2), 2))