There are 15 slate rocks, 20 pumice rocks, and 10 granite rocks randomly distributed in a certain field. If 2 rocks are to be chosen at random and without replacement, what is the probability that both rocks will be slate rocks?
(A) 1/3
(B) 7/22
(C) 1/9
(D) 7/66
(E) 2/45
ANS: D
為什麼不是(E) ? 我錯誤的想法是取二個,那就是總數取2個=> 2/45
(D)的答案怎麼算都算不到.... 請問如何算到 7/66 ? 謝謝指教