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← JEE Advanced 2024 · Paper 2
MathematicsNumerical+3Question 14 of 51

JEE Advanced 2024 · Paper 2, Mathematics question 14

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PARAGRAPH "I"

Let S={1,2,3,4,5,6}S = \{1, 2, 3, 4, 5, 6\} and XX be the set of all relations RR from SS to SS that satisfy both the following properties:

  1. RR has exactly 6 elements.
  2. For each (a,b)∈R(a, b) \in R, we have ∣a−b∣≥2|a - b| \ge 2.

Let Y={R∈X:The range of R has exactly one element}Y = \{R \in X : \text{The range of } R \text{ has exactly one element}\} and Z={R∈X:R is a function from S to S}Z = \{R \in X : R \text{ is a function from } S \text{ to } S\}.

Let n(A)n(A) denote the number of elements in a set AA.

If n(X)=mC6n(X) = {}^{m}C_6, then the value of mm is _____.

Answer and worked solution

Official answer: 19.99

S×SS\times S has 3636 ordered pairs. Remove those with ∣a−b∣<2|a-b|<2:

  • 66 pairs with a=ba=b
  • 2⋅5=102\cdot5=10 pairs with ∣a−b∣=1|a-b|=1

That leaves 2020 pairs with ∣a−b∣≥2|a-b|\ge2. A relation in XX is any 66-element subset of these, so
n(X)=20C6  ⟹  m=20.n(X)={}^{20}C_6\implies m=20.