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Aptitude Day 110

Answer of Day 109: 7300

Hey friends... We're gonna see problems on Permutations and Combinations from tomorrow. So let's look at those formulae today.

Formulae:

  • Factorial Notation n! = n(n-1)(n-2).... 3.2.1.
  • 0! = 1.


Permutations:

  • nPr = n! / (n-r)!
  • All permutations made with the letters a,b,c  by taking two at a time are (ab, ba, ac, ca, bc, cb).
  • All permutations made with the letters a,b,c taking all at a time are (abc, acb, bac, bca, cab, cba).

Combinations:

  • nCr = n(n-1)(n-2).... to r factors / r!
  • nCr = nCn-r 
  • If we want to select 2 out of 3 girls A, B and C. Then the possible selections are AB,BC and CA.
  • All the combinations formed by a,b,c taking two at a time are ab, bc and ca.
  • The only combination that can be formed of 3 letters a,b,c taken all at a time is abc.  
  • ab and ba are two different permutations but they represent the same combination.
  • nCn =1.
  • nCo = 1.

           You'll be able to get a clear idea while we're solving the problems. 

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