问一道prep的概率题

a certain basket contains 10 apples, 7 of which are red and 3 of which are green. if 3 different apples are to be selected at random from the basket, what is the probability that 2 of the apples sellected will be red and 1 will be greed?

7/10 * 6/9 * 3 / 8 = 7/40

就知道在后面还应该乘以一个排列或组合的。。。。。就是想不出来。。。。。。 越紧张越想不起来。。。。。

求救阿！！！谢谢了！

答案是21/40。

明白了．．．．谢谢！！！

所以我应该在我的做法后乘以３。。。。。谢谢您！