Question

There is a bag filled with 2 blue, 4 red and 5 green 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 2 the same colour?​

Answer 1

Answer: 45/121

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Work Shown:

2 blue, 4 red, 5 green

2+4+5 = 11 total

P(blue) = probability of getting one blue marble

P(blue) = (number of blue)/(number total)

P(blue) = 2/11

A = P(2 blue) = (2/11)*(2/11) = 4/121

P(red) = 4/11

B = P(2 red) = (4/11)*(4/11) = 16/121

P(green) = 5/11

C = P(2 green) = (5/11)*(5/11) = 25/121

Add the values of A,B, and C to get the probability of getting two of the same color

A+B+C = 4/121+16/121+25/121 = 45/121

This is approximately equal to 0.3719 which is roughly a 37.19% chance of happening

Side note: we can add A,B, and C because they are mutually exclusive events.