What is the probability that a ball now drawn from the first bag is white?One bag contains 4 white balls and 3 black balls, and a second bag contains 3 white balls and 5 black balls. One ball is...

What is the probability that a ball now drawn from the first bag is white?

One bag contains 4 white balls and 3 black balls, and a second bag contains 3 white balls and 5 black balls. One ball is drawn at random from teh second bag and is placed unseen into the first bag.

Asked on by jenny321

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nathanshields | High School Teacher | (Level 1) Associate Educator

Posted on

First bag: WWWWBBB
Second bag: WWWBBBBB

Now, before switching any balls, the probability of drawing a white ball from the first bag is 4/7.  Since there are more black balls than white balls in the 2nd bag, transfering a ball from the 2nd bag to the 1st bag should make the 4/7 slightly lower, right?  Just something to think about before starting.

Let's take each case separately:

Case 1: Transfered ball is black.  Now the first bag would have WWWWBBBB, so probability of drawing white is 50%.

Case 2: Transfered ball is white.  Now the first bag would have WWWWWBBB, so probability of drawing white is 5/8.

But what is the probability of case 1? The probability of drawing a black ball from the 2nd bag. (5/8)

Similarly, the probability of case 2 is 3/8.

We multiply the probabilities for each case, since the events are independent:

P(transfereing a black ball AND picking a white ball): `5/8*1/2` =5/16

P(transfering a white ball AND picking a white ball): `3/8*5/8` =15/64

Since either option is acceptable, we add their probabilities:

`5/16+15/64=(20+15)/64=35/64`

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