Answer: 17/24
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Step-by-step explanation:
We have these four cases or possible outcomes
- Case 1) We select 0 black marbles and 3 red marbles.
- Case 2) We select 1 black marble and 2 red marbles.
- Case 3) We select 2 black marbles and 1 red marble.
- Case 4) We select 3 black marbles and 0 red marbles.
Let's calculate the probability for case 4.
There are 7 black marbles out of 10 total. The probability of picking black is 7/10. If no replacement is made, then 6/9 is the probability of picking black again (subtract 1 from the numerator and denominator separately). Finally, 5/8 is the probability of getting black a third time.
The probability of getting 3 black marbles in a row is
(7/10)*(6/9)*(5/8) = (7*6*5)/(10*9*8) = 210/720 = 7/24.
That fraction 7/24 means that if you had 24 chances, then you expect about 7 of them will lead to getting three black marbles in a row (aka case 4). Therefore, 24-7 = 17 occurrences are expected where we get cases 1 through 3 occur in some fashion (pick one case only).
Notice how cases 1 through 3 encapsulate the phrasing "at most 2 black marbles" which is another way of saying "2 black marbles is the highest we can go".
So that's why the answer is 17/24.