[求助]PREP数学题

huangxiaoyan

There are 15 slate rocks, 20 pumice rocks, and 10 granite rocks randomly in a certain field. If 2 rocks are to be chosen at random and without replacement, what is the probobility that both rocks will be slate rocks?

答案是7/66

newaaron

15/45*14/44=7/66

愁予

without replacement说明第一次是在45个里面选，概率是15/45=1/3。第二次是在剩下的44个里面选，概率是14/44=7/22。两次乘法原理：1/3*7/22=7/66

huangxiaoyan

多谢

李傻傻

用排解组合就是:C(15,2)/C(10+20+15,2)=66/7

李傻傻

错了,7/66