HOMEWORK 5: SOLUTIONS
Abstract. The problems are taken from the textbook. The numbers are referred to in the beginning. You do not need to use continuity correction
for the exams. So, you can ignore those comments in t
...
HOMEWORK 5: SOLUTIONS
Abstract. The problems are taken from the textbook. The numbers are referred to in the beginning. You do not need to use continuity correction
for the exams. So, you can ignore those comments in the following solutions.
Problem 1. We choose 500 numbers uniformly at random from the interval
[1:5; 4:8].
(a) Approximate the probability of the event that less than 65 of the numbers start
with the digit one.
(b) Approximate the probability of the event that more than 160 of the numbers
start with the digit 3.
Solution. You do not need to use continuity correction for the exams.
Date: Nov 7, 2018.
12
=
( ([0, ]) = 0, ([ , ]) = 13)
P(N([0, 4]) = 13)
=
P(N([0, 1]) = 0) · P(N([1, 4]) = 13)
P(N([0, 4]) = 13)
=
e!4 · e!121213/13!
e!161613/13!
= ✓3 4◆13
⇡ 0.02376.
4.16. (a) Denote by S the number of random numbers starting with the digit 1.
Note that a number in the interval [1.5, 4.8] starts with 1 if and only if it is in
the interval [1.5, 2). The probability that a uniformly chosen number from the
interval [1.5, 4.8] is in [1.5, 2) is equal to p = 4.80!.51.5 = 33 5 . Assuming that the
500 numbers are chosen independently, the distribution of S is binomial with
parameters n = 500 and p.
To estimate P(S < 65) we use normal approximation. Note that E[S] =
np = 500 · 33 5 ⇡ 75.7576 and Var(S) = np(1 ! p) ⇡ 64.2792. Hence
P(S < 65) = P ✓Sp!6475 .2792 .7576 < 65p!6475 .2792 .7576◆ ⇡ P ✓Sp!6475 .2792 .7576 < !1.34◆
⇡ !(!1.34) = 1 ! !(1.34) ⇡ 1 ! 0.9099 = 0.0901.
Note that P(S < 65) = P(S 64). Using 64 instead of 65 in the calculation
above gives 1 ! !(1.47) ⇡ 0.0708. If we use the continuity correction then we
Solutions to Chapter 4 97
need to use 64.5 instead of 65 which gives 1 ! !(1.4) ⇡ 0.0808. The actual
probability (evaluated numerically) is 0.0778.
(b) We proceed similarly as in part (a). The probability that a given uniformly
chosen number from [1.5, 4.8] starts with 3 is q = 31.3 = 10 33. If we denote
the number of such numbers among the 500 random numbers by T then T ⇠
Bin(n, q) with n = 500.
Then
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