What is the probability of the drawn ball being blue?
Solution. Let A be the event of drawing a blue ball. After a ball was drawn from the second box and placed into the first, two collections of balls have turned out in the first box: 8 blue and 7 red balls it contained prior to replacement;
one ball replaced from the second box.
The probability of appearance of a blue ball belonging to the first collection is and from the second collection, .
The probability that he randomly drawn ball belongs to the first collection is and to the second, .