Two issues here:

1. We are actually not looking at but rather — i.e. the probability of is the probability of AND working so the solution in the notes is sound.

2. Your solution is nearly OK… to fix it you need to use the alternative solution below. The problem is that A OR B, in maths, includes

A and B

A and not B

B and not A,

it is an inclusive rather than exclusive OR (as we sometimes see in English, i.e. “do you want chips or spuds with your dinner”).

Therefore, to fix your solution we need to look at

.

Regards,

J.P.

Just on your notes on P. 106 and P. 107; I have attached a file, on top was what I took down off the board and on the bottom is what I have made out as the answer.

Now I can’t seem to understand for why you would have probability of : surely if either fail here then a the system will fail as they are in series?

And if so then is the answer actually is more reliable as shown in the attachment?

Thanks.

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