It seems too simple so I must be wrong! 40% or 0.4.

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You got it! Yaaaay!

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Probablility is nothing but the ratio:

Total Target Outcomes

Total of All Possible Outcomes

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In this case, there are a total of 50 marbles you could select at random (there are 50 marbles in the jar).

So that's the denominator.

Then there are 20 red marbles, any one of which is a "target" or a desired selection.

So that's the numerator. Consequently, you simply plug in the numbers and do the arithmetic:

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20

50

= 2/5 = 0.4 = 40%

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Likewise, if you wanted a blue marble, it would be: 30/50 = 3/5 = 0.6 = 60%

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Then to figure for multiple selections, you compute the probability for each draw, and multiply the probabilities together.