S2 help

In c, why did the mark scheme approximate to poisson?
I approximated to normal since n is large and p is large as well and np>10

You need to check the criteria for approximating a binomial with a normal distribution.

As well as a restriciton on np, there is also a restriction on nq, that is n(1-p).

A cadet fires shots at a target at distances ranging from 25 m to 90 m. The probability of
hitting the target with a single shot is p. When firing from a distance d m, p d = − 3
200 90( ).
Each shot is fired independently.
The cadet fires 10 shots from a distance of 40 m.
(a) (i) Find the probability that exactly 6 shots hit the target.
(ii) Find the probability that at least 8 shots hit the target.
(5)
The cadet fires 20 shots from a distance of x m.
(b) Find, to the nearest integer, the value of x if the cadet has an 80% chance of hitting
the target at least once.
(4)
The cadet fires 100 shots from 25 m.
(c) Using a suitable approximation, estimate the probability that at least 95 of these shots
hit the target


In c, why did the mark scheme approximate to poisson?
I approximated to normal since n is large and p is large as well and np>10

what do you mean by n(1-p)?

p is the probability of success in the binomial. n is the number of trials.

You really should be familiar with this. As I said previously, check the criteria for approximating a binomial with a normal distribution.

I know that I meant why what do you mean by "there is also a restriction on nq, that is n(1-p)."