First slide
Combinations
Question

 A seven digit number is in the form of abcdefg(g,f,e, etc. are digits at units, tens, 

 hundreds place etc.) where a<b<c<d>e>f>g . Then the number of such possible numbers is

Moderate
Solution

The minimum value of  d will be 4

 i) If d=4 , then the number of seven digit numbers possible is 3C34C3=4

 [as a,b,c can be chosen from 1,2 or 3 and similarly e,f,g can be chosen from 0,1,2 or 3 ] 

 ii) If d=5 , then the number of seven digit numbers possible is 4C35C3=40

 [as a,b,c can be chosen from 1,2,3 or 4 and similarly e,f,g can be chosen from 0,1,2,3 or4 ]

 iii) If d=6 , then the number of seven digit numbers possible is 5C36C3=200

 [as a,b,c can be chosen from 1,2,3,4 or 5 and similarly e,f,g can be chosen from 0,1,2,3,4 or 5]

 iv) If d=7 , then the number of seven digit numbers possible is 6C37C3=700

 [as a,b,c can be chosen from 1,2,3,4,5 or 6 and similarly e,f,g can be chosen from 0,1,2,3,4,5,6 or 7] 

 v) If d=8 , then the number of seven digit numbers possible is 7C38C3=1960

 [as a,b,c can be chosen from 1,2,3,4,5,6 or 7 and similarly e,f,g can be chosen from 0,1,2,3,4,5,6 or 7] 

 vi) If d=9 , then the number of seven digit numbers possible is 8C39C3=4704

 [as a,b,c can be chosen from 1,2,3,4,5,6,7 or 8 and similarly e,f,g can be chosen from 0,1,2,3,4,5,6,7 or 8]

 Total number of numbers is 4+40+200+700+1960+4704=7608 

 Therefore, the correct answer is (3) . 

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