I'm sorry,wrong,too old to be right soon.

碰對的幾率每題是1/3,4題碰對2題幾率是C(4,2)*(1/3)^2*(2/3)^2=24/81
4題碰對3題幾率是C(4,3)*(1/3)^3*(2/3)^1=8/81
4題碰對4題幾率是C(4,4)*(1/3)^4=1/81
第二次考試通過的幾率為和:33/81=11/27

需要重考3次，方能保證通過.窮舉法可得.

所有跟帖： 

• let me try 第二次考試通過的幾率. -皆兄弟也- ♂ (442 bytes) () 07/31/2010 postreply 14:14:26

• 此答案應是第一次考試四題對兩題以上通過的幾率. -皆兄弟也- ♂ (0 bytes) () 07/31/2010 postreply 14:33:02

• you have to according nacl's second remind. -jinjing- ♀ (431 bytes) () 07/31/2010 postreply 15:05:44

• That might be correct. -皆兄弟也- ♂ (0 bytes) () 07/31/2010 postreply 15:12:20

• Yes,you are smart,I can use formula only. -jinjing- ♀ (0 bytes) () 07/31/2010 postreply 15:33:18

• just have fun. -皆兄弟也- ♂ (0 bytes) () 07/31/2010 postreply 15:45:34