Let A be the set of 4 digit number a_1a_2a_3a_4, where a_1<a_2<a_3<a_4, then n(A) is equal to

JEE Main Mathematics question

Let be the set of digit number , where , then is equal to

Answer: (B)

A four-digit number cannot begin with , and forces to be the smallest digit, so all four digits come from

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Question ID VK-020670