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# S1 help! watch

1. Hi,

I came across some questions where I wasn't able to understand why. Please help me with these questions, your help will be greatly appreciated!

1) Zara's friend, Wei, also attends the club's practice sessions. The probability that Wei arrives late is 0.9 when Zara arrives late, and is 0.25 when Zara does not arrive late. (* p(zara arrives late)= 0.2)
a)either zara or wei, but not both, arrives late.

--> To get to the answer, I found 2 ways of solving.
1) 1- p(both late) - p(both early) = 0.22
2) p(Zara late and Wei early) + p(Wei late and Zara early)= 0.22

so for 2) it's (0.2x0.1) + (0.25x0.8)
but why doesn't it give the same answer when I do (0.9x0.8)+(0.75x0.2) ?

Is there a specific way of solving these types of questions?

2) Some of the questions ask why it is normally distributed and I come across answers like 'sample size more than 30, CLT etc.'

How do we know when to use these answers? What tells us that CLT is used?

Thank you!
2. It's conditional remember, so when Zara arrives early the probability of Wei arriving late changes, double check that last calculation and you'll see it.

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