If n items are arranged two at a time, the number obtaine...
If n items are arranged two at a time, the number obtained is 20. Find the value of n.
5
10
15
40
Correct answer is A
nP2=n!(n−2)!=20
n(n−1)(n−2)!(n−2)!=20
n(n−1)=20⟹n2−n−20=0
n2−5n+4n−20=0
n(n−5)+4(n−5)=0
n=5 or -4
n=5
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