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Probability of taking a white ball from black container = 1/5
Then putting in black container and probability of taking a wihte ball from it = 1
Therefore tatal probability = 1/5

Probability of taking a black ball from black container = 4/5
Then putting in black container and probability of taking a white ball from it = 3/4
Therefore total probability = 3/5

Hence probability of white ball is = 3/5 + 1/5 = 4/5

1. probality of picking 'white ball' from 'black container' is 1/5 and 'black ball' is 4/5.
2. 'white container' contains 3 existing 'white balls' + 1/5 new 'white balls'
and 4/5 new 'black balls'.
3. probabilty of the ball could be white is (3 + 1/5)/ (3+ 1/5 + 4/5)
= (16/5)/(4)
= 4/5

4/5 is right
3/4+1/4*1/5

1. probality of picking 'white ball' from 'black container' is 1/5 and 'black ball' is 4/5.
2. 'white container' contains 3 existing 'white balls' + 1/5 new 'white balls'
and 4/5 new 'black balls'.
3. probabilty of the ball could be white is (3 + 1/5)/ (3+ 1/5 + 4/5)
= (16/5)/(4)
= 4/5

Wow sachin.magdum's method is ... weird.

Probability of picking a black ball = (4/5)*(1/4) = 1/5 (first term is from black container, second term from white container)

Thus probability of picking a white ball is 1 - 1/5 = 4/5.

Nipun's method looks fine.

Yeah. Even I got 4/5.

Yes, I got 4/5 too.

:-) tonnes of same answer.... Seems everyone is having good concept in Probability....

Hey Sachin... Urs is a different method actually, please correct me if I understood your method correctly.....
It's based on basic prob definition. p(x)=(number of x occurrences)/(total occurrences).
3+1/5 = White ball occurrence
3+1/5+4/5 = Total occurrences....
So, P(white ball) = {3+1/5}/{3+1/5+4/5}
Nice, I never thought of solving this problem by this method.. Would have taken Nipun's or fiturus's method

use simple bayes theorem