Question

There is a bag filled with 5 blue and 4 red marbles. A marble is taken at random from the bag, the colour is noted and then it is replaced. Another marble is taken at random. What is the probability of getting exactly 1 blue?

Answer 1

you would add blue and red together so 5+4 which is 9, you would then need to answer this as a fraction which would be 5/9

Answer 2

Answer:

40/81.

Step-by-step explanation:

Prob(Picking a blue on one pick) = 5/(5+4) = 5/9.

Prob(Picking a red on one pick) = 4/(5+4) = 4/9.

Required probability  is either the first pick is blue OR the second pick is blue. (The other probability on one pick is of course a red marble.)

Probability of at least 1 blue = P(red)*P(blue) + P(blue)*P(red)

= 4/9 * 5/9 + 5/9*4/9

= 20/81 + 20/81

= 40/81.

Another way of solving this is by using a tree diagram.