pp-T1-DS158

[Huang Hsin-Yi]

158.
Each of the 25 balls in a certain box is either red, blue, or white and has a number from 1 to 10 painted on it. If one ball is to be selected at random from the box, what is the probability that the ball selected will either be white or have an even number painted on it?

(1) The probability that the ball will both be white and have an even number painted on it is 0.

(2) The probability that the ball will be white minus the probability that the ball will have an even number painted on it is 0.2.

Ans:E

請問該從那裡思考起？

[Huang Hsin-Yi]

終於問到對岸高手了

問P (either white or even)= P (white)+P(even)- P(both)
(1)只知 P(both)
(2)只知P(white)-P(even)

therefore insufficient

[lovenarimiya]

我解的時候是這樣想的:

設A=P(white)
B=P(even)
A*B= P(both)

則由(A-B)^2+4AB開平方根
可以推導出A+B的數值
兩個選項合併可以求得答案

請問這樣想錯在哪?

謝謝!!

[Huang Hsin-Yi]

機率問題和代數 我連不太起來
所以我也不知你那裡錯了