wrote in message
...
[...] the chance is actually only 75%.

How so?

The probability of both engines failing is .25, I agree, but I'm
talking a failure of either engine.

I am too. What chance do YOU think you have of having a failure of either
engine, if not 75% (in this example)?

If the chance of an engine failure is 50% (0.5), then the chance of either
engine failing when you have two engines is 1-(0.5)*(0.5). 75%.

The probability of both engines failing is indeed only 25%. The probability
of EITHER engine failure is 75%.

You need to do the subtraction because the chance of an engine failure is
actually the opposite of the chance of completing a flight without an engine
failure. To make the flight successfully without either engine failing
requires BOTH engines to not fail, and the way to calculate that is to
multiply the chances of each engine failing (which in this case is just two
engines, with identical chances).

The chance of you completing the flight without a failure is 25% (50% *
50%), so the chance of an engine failure on the flight is 75%.

Pete