(a) Two items are selected at random from four items labelled (p, q, r, s).
(i) List the sample space if sampling is done (1) with replacement ; (2) without replacement.
(ii) Find the probability that r is at least one of the two objects selected : (1) in a(i)1 ; (2) in a(i)2.
(b) How many whole numbers from 100 to 999 are divisible by (i) 4 ; (ii) both 3 and 4?
(a)(i)(1) With replacement
A
B
Scientific Diagram
Since an item is replaced after the first selection, there are \(4 \times 4 = 16\) possible ordered outcomes:
p
q
r
s
p
pp
pq
pr
ps
q
qp
qq
qr
qs
r
rp
rq
rr
rs
s
sp
sq
sr
ss
\(S=\{pp,pq,pr,ps,qp,qq,qr,qs,rp,rq,rr,rs,sp,sq,sr,ss\}\).
(2) Without replacement
There are \(4 \times 3=12\) possible ordered outcomes:
\(S'=\{pq,pr,ps,qp,qr,qs,rp,rq,rs,sp,sq,sr\}\).
(a)(ii)(1) With replacement
The outcomes containing at least one \(r\) are:
\(\{pr,qr,rp,rq,rr,rs,sr\}\).
Therefore,
\[
P(\text{at least one }r)=\frac{7}{16}.
\]
(2) Without replacement
The outcomes containing \(r\) are:
\(\{pr,qr,rp,rq,rs,sr\}\).
Therefore,
\[
P(\text{at least one }r)=\frac{6}{12}=\frac{1}{2}.
\]
(b)(i) Numbers from 100 to 999 divisible by 4
The multiples of 4 are:
\(100,104,108,\ldots,996\).
\[
996=100+4(n-1)
\]
\[
996=100+4n-4
\]
\[
996=96+4n