Let A be the set of 4-digit numbers a1 a2 a3 a4 where a1> a2> a3> a4, then n(A) is equal to

Q: Let A be the set of 4-digit numbers a₁ a₂ a₃ a₄ where a1> a2> a3> a4, then n(A) is equal to

(A) 126

(B) ¹⁰C₄

(D) none of these

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Sol: Any selection of four digits from the ten digits 0, 1, 2, 3, . . . , 9 gives one such number.

So, the required number of numbers = ¹⁰C₄ = 210 .

Hence (B), (C) are the correct answer.