NTA Abhyas JEE Main2020MathematicsPermutation and CombinationPractice
The number of 7 digit integers a b c d e f g , where a < b < c < d > e > f > g such that a , b , c , d , e , f , g ∈ 1,2 , 3 , . . . . , 9 , are
Options
- A700
- B20
- C720
- D800
Correct answer
C. 720
Step-by-step solution
Case 1 : d = 7 6 C 3 × 3 C 3 = 20 Case 2 : d = 8 7 C 3 × 4 C 3 = 140 Case 3 : d = 9 8 C 3 × 5 C 3 = 560 Total = 20 + 140 + 560 = 720