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.