Q: Let A be the set of 4-digit numbers a_{1} a_{2} a_{3} a_{4} where a1> a2> a3> a4, then n(A) is equal to

(A) 126

(B) ^{10}C_{4}

(C) 210

(D) none of these

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 = ^{10}C_{4} = 210 .

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