Statistics 2

(i) x_bar=4.3,n=40,s=1.2

The standard deviation of the typos is 1.2/√40=0.1897

(ii) The 96% C.I. is
(4.3-2.055*0.1897<u<4.3+2.055*0.1897)
(3.9074<u<4.6926)

(iii) The result in (ii) means that there is a large probability that John have made 4 to 5 mistakes per page and it should be a good choice for him to check the whole dissertation once more.