inga wrote:

There are 5 blue balls and 7 red balls in the 1st box and 11 blue and 4 red in the 2nd one. Ann takes 1 ball out of a box. What is the probability that the chosen ball is blue? (we don't know out of which box Ann's taken the ball)

Don't have OA. Not sure that numbers are correct. Just faced such a question in one of the tests I have taken.

What about this approach

plz if i am wrong tell me why

the blue marble can be taken either from the first box or the second

prob of a blue first box =5/12

prob of a blue second box 11/15

5/12+11/5 PROB to have a blue one from either box

thanks