Let X denote the number of elements in set X . Let S = 1 ,   2 ,   3 ,   4 ,   5 , &#160…

Let X denote the number of elements in set X. Let S=1, 2, 3, 4, 5, 6 be a sample space, where each element is equally likely to occur. If A and B are independent events associated with S, then the number of ordered pairs A, B such that 1B<A, equals

Solution

Since A and B are independent events
PAB=PAPB
ABS=AS·BS
S AB=A·Bx=number of element in set
S=6
6AB=AB
6nAB=nAnBset x=nx
Case I:
nAB=1
nAnB=6
a nA=6, nB=1Number of ways=C6 6×C1 6=6

b nA=3, nB=1  Number of ways=C36×C13×C25=180


Case II:
nAB=2
nAnB=12
a nA=6nB=2Number of ways=C6 6C2 6=15
b nA=4nB=3Number of ways=C4 6×C2 4×C1 2=180
Case III:
nAB=3
nAnB=18
a nA=6,nB=3 Number of waysC6 6×C3 6=20
Case IV:
nAB=4
nAnB=24
a nA=6nB=4 Number of waysC6 6×C4 6=15
Case V:
nAB=5
1nAnB=30
a nA=6nB=5 Number of waysC6 6C5 6=6
Total ordered pairs =422

Asked in: JEE Advanced 2019 (Paper 2)

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