# Category: Quantitative Aptitude

## Permutation & Combination

Combination A. Useful Relations (i). 0!=1!=1 (ii). Con=n!0!*n!=10!=1 (iii). n!=n*(n−1)! (iv). Prn=n!(n−r)! (v). (n+1).Prn=Pr+1n+1 (vi). Crn=Prnr! (vii).Crn=Cn−rn (viii).Cr−1n−1+Crn−1=Crn 2. Fundamental Principle of Counting Let us now solve the problem mentioned in the introduction. We will write...