Answer: 1/12
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Step-by-step explanation:
There are 3 red out of 9 total
The probability of picking red at random is 3/9 = 1/3
If the first marble is not put back, then the chances of picking red again are now 2/8 = 1/4
Notice how I subtracted 1 from the numerator and denominator of 3/9 to get to 2/8. This is because the red count and the total count both go down by 1 each.
Now multiply the fractions mentioned:
(3/9)*(2/8) = (1/3)*(1/4) = (1*1)/(3*4) = 1/12
If you did this trial 12 times, then you should expect to get two reds about once. That's one way to interpret the probability of 1/12.
Side note: 1/12 = 0.0833 = 8.33% approximately