## Question

Let *A *be the set of 4-digit numbers a_{1}a_{2}a_{3}a_{4} where a_{1} > a_{2} > a_{3} > a_{2} then *n *(*A*) is equal to :

### Solution

210

Select 4 digits out of 10 in ^{10}C_{4} ways.

As order is fixed for arrangements. [∴ a_{1} > a_{2} > a_{3} > a_{4}]

Hence No. of ways

= (No. of ways to select ) × 1

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