It depends on how well you pack it, how good a condition it is in, and on how correctly you use it.
In practice, the probability is very low. About one in every 100,000 jumps in the US results in a fatality, a probability of about 0.00001, but that's not just due to parachute failure. One unconfirmed estimate is that one in a thousand (0.001) main chute deployments result in a malfunction, but it is true that very few of those result in a fatality, primarily because there is a reserve chute that is maintained and packed under different, more stringent, requirements.
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If the probability of succeeding is 2%, then the probability of failing is 98%. The probability, then, of failing 75 times is 98%75, which is 22%.
Probability of failing to reject a false null hypothesis.
Assuming the alternator's failures are unrelated, the probability of both failing is the product of the individual probability, or 0.022, or 0.0004. The duration of the flight does not matter.
In hypothesis testing, this is the probability of failing to reject a false null hypothesis.
Any time there are two options, heads or tails when you flip a coin for example, the probability is 1/2, that the result will be either one option, or the other. The expected result when the coin lands is a 1/2 probability that it will be heads, and a 1/2 probability that it will be tails. What "1 out of 2 failing" means is that for every two students that take an exam, for example, one of them will fail. Of course, it also means, that 1/2 will pass.