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Aptitude Day 111

Problems on Permutations and Combinations:

1. 20!/18!
              = 20*19* 18! / 18! = 20*19 = 380.
2. 10P4 
           nPr  =  n!/(n-r)! 

             = 10!/ (6!) = 10*9*8*7*6!/ 6! = 5040.
3. 5P5 
             = 5!/0! = 5*4*3*2*1 /1 = 120.
4. 30C2
               nCr = n(n-1)(n-2).... to r factors / r!

             = 30*29/1*2
             = 435. 
5. 50C48 
               nCr = nCn-r

              = 50C50-48 = 50C2 = 50*49/ 1*2 = 1225.

6. 30C30 
               nPn = 1` 

              = 1. 



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