Probability (URGENT, O level in a few hours' time!)

anpanman SG

Bag A contains 4 white marbles and 5 black marbles. Bag B contains 7 white marbles and 3 black marbles. A marble is selected from Bag A and placed in B without its colour noted. A marble is then drawn from B. Using a 'Tree' diagram, find the probability that the marble drawn from B is black

my working

4/9 X 3/11 + 5/9 X 4/11 = 32/99

ans given is 22/99.

anpanman SG

2. Two bags A and B, contain some red and blue counters. bag A contains 8 red counters and 4 blue counters. B contains 3 red counters and 5 blue counters. A bag is chosen by tossing a coin. If the coin shows a head, bag A is chosen, otherwise bag B is chosen. A counter is then selected at random from the chosen bag.

Calculate the probability of getting a Head on a coin and drawing a blue counter

my ans : 1/2 X 1/3 = 1/6

is it alright?

dkcx SG