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That is the problem. I also marked letter (D). I think that official answer (E) is wrong.

Hero8888
CursoFDX
The probability of success in a field event is the ninth part of the probability of failure. Successive and independent trials are held until success occurs for the first time. In these circumstances, the expected number of failures that should occur until the first success occurs is equal to:

(A) 2
(B) 3
(C) 6
(D) 9
(E) 10

Let Failure = x, so Success= x/9. Since S+F=1 --> x + x/9 = 1 --> F=9/10 and S=1/10

{9F}+{1S}=10. I got max 9 Failures before 1 Success, why 10 F (E)? The Q asks how many F not trials why 10 and not 9???
Thanks.

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