f5 maths !!!

2008-09-05 6:35 am

Bag A contains 3 white balls and 2 red balls
Bag B contains 2 white balls and 3 blue balls
Bag C contains 1 red ball only

After drawing a ball, Betty puts the ball back to the original bag. Then she draws another ball from a randomly selected bag. What is the probability that both balls drawn are red and they come from the same bag ?

ans:29/225

why?

有時間請到我的發問,我仲有一條唔明....

回答 (1)

Gabriella Montez

2008-09-05 6:59 am

✔ 最佳答案

P ( 2 red balls are drawn from A with replacement )
= ( 1 / 3 )( 2 / 5 )( 1 / 3 )( 2 / 5 )
= 4 / 225
P ( 2 red balls are drawn from B with replacement )
= ( 1 / 3 )( 0 )( 1 / 3 )( 0 )
= 0
P ( 2 red balls are drawn from C with replacement )
= ( 1 / 3 )( 1 )( 1 / 3 )( 1 )
= 1 / 9
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P ( 2 red balls are drawn from the same bag )
= P ( 2 red balls from A ) + P ( 2 red balls from B ) + P ( 2 red balls from C )
= 4 / 225 + 0 + 1 / 9
= 29 / 225

參考: My Maths Knowledge

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