help with AS maths S2 ?

OK so it's question 2 that im stuck on. The question always starts with using a suitable distribution ...
It's OCR MEI .
http://www.mei.org.uk/files/papers/S2_2014_June.pdf 2 (vi) how do i know if its X - N(n,p) or Normal(...) ?

Takes practice. *usually* the words average rate suggest Poisson

Also, theoretically the poisson can go up to very high numbers (like 1,000,000) but the probability of that is.. 0, or as close to 0 as you can get (use P(X=1 million) for a poisson distribution with a rate of like 10 ), but in this situation it would make sense if you had say 100 defects in 1 square metre, whereas it wouldn't make sense in another situation, whereas if you had 100 people in a town, you thought the probability of winning the lottery was 1 in 100 and 5 people one it, it wouldn't make sense to use poisson as you could have (a very, very, very low chance) of having 150 winners out of 100 people.

So when n is large and p is small use poisson but when similar then Normal() ?

Oh, I seem to have misunderstood your question - sorry!

If n is such that it can't be read off the tables then that is usually when you go to normal. (Are you allowed to approximate a poisson with a binomial in OCR, or are you thinking of the other way around?)