The total number of possible outcomes for a single toss of a pair of dice N=6⋅6=36.
The favorable outcomes (Renzo gets 9) are (3, 6), (4, 5), (5, 4), (6, 3); n=4.
The probability of getting a 9 for each toss is p=364=91.
The probability of getting another sum is q=1−91=98.
We have a Bernoulli trial - exactly two possible outcomes, "success" (Renzo gets a 9) and "failure" (Renzo gets another sum) and the probability of success is the same every time the experiment is conducted (Renzo tosses a pair of dice).
The probability of each result
P(X=k)=(nk)⋅pk⋅qn−k==(5k)⋅(91)k⋅(98)5−k==k!⋅(5−k)!5!⋅(91)k⋅(98)5−k.
The sought probability that Renzo gets at least three 9:
P(X≥3)==P(X=3)+P(X=4)+P(X=5)==3!⋅2!5!⋅(91)3⋅(98)2+4!⋅1!5!⋅(91)4⋅(98)1++5!⋅0!5!⋅(91)5⋅(98)0==0,011533.
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