RRB Group D Permutation & Combination

Permutation deals with ARRANGEMENTS where order matters. If you arrange 3 people in a line, ABC is different from BAC. Combination deals with SELECTIONS where order does not matter. If you select 3 people for a team, ABC is the same as BAC

nPr = n!/(n-r)! where n is total items, r is items to arrange 2

nCr = n!/(r!(n-r)!) where n is total items, r is items to select 3

n! = n × (n-1) × (n-2) × ... × 1, and 0! = 1 4. When all items are arranged: nPn = n! 5

(n-1)! for clockwise and anticlockwise same

Use nCr = nC(n-r) to reduce calculations. For 10C8, calculate 10C2 instead. 2

For repeated letters, use n!/repetition factors 3

Use complement method (Total - Unwanted) Worked Example 1: In how many ways can 5 people sit in a row

If the question talks about positions, ranks, or arrangements, use permutation. If it talks about selection, teams, or groups, use combination. For example, 'selecting 3 students' is combination, but 'arranging 3 students in first, second, third position' is permutation.