A certain airplane has two independent alternators to provide electrical power. The probability that a given alternator will fail on a 1-hour flight is .02. What is the probability that one or the other will fail?
okay, digging back into the depths of my memory of probability & statistics...
the probability that 1 or the other will fail is 1 minus the probability that neither will fail.
Probability that each alternator will NOT fail is .98
Statistically independent events, so, you multiply these together.
1 - (.98 x .98) = .0396
Way, way back when I was young, I entered Navy flight training on the T-28 Trojan.
...which had a 1475 horsepower piston engine, with 2 alternators, just as described here, for safety. (They called them "magnetos" instead of alternators, but same thing.)
..because the probability of BOTH of them failing is only .02 x .02 = .0004