Porisma

A hint to get started

Keep the square-root growth in the large-sample tail

X∼N(3,4)X\sim N(3,4), with mean 3 and variance 4. S sums 100 independent observations of X; T sums a further 300 independent observations. Give numerical answers to 3 decimal places.

(i) Find P(S>310)P(S>310).

(ii) Find P(3S>50+T)P(3S>50+T).

(iii) N sums n independent observations of X. State the approximate value of P(N>3.5n)P(N>3.5n) as n becomes very large, with justification.

[1989/II/9]

CJC · Discussion · Question 13

The gap from the mean grows like n, while the sum’s standard deviation grows like√n.

Read the worked solution