What is the probability of choosing a red marble or a green marble out of a bag containing 5 blue marbles 3 red marbles and 1 green marble?

Pr = ways_of_success/total_ways

Pr(Red) = 3/(5+3+1) = 3/9

Pr(Green) = 1/9

As they are independent:

Pr(Red or Green) = Pr(Red) + Pr(Green) = 3/9 + 1/9 = 4/9

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Alternatively, as there are only three colours, selecting either Red or Green is the same as not selecting Blue:

Pr(Red or Green) = Pr(not Blue) = 1 - Pr(Blue)

→ Pr(Red or Green) = 1 - 5/9 = 4/9