Core5 mapped NDA questions
Build the idea
Build exam-ready understanding by connecting the core idea, formulas or facts, worked example and previous NDA questions. Focus on why each rule works, then test it under timed conditions.
Concept
Permutations count ordered arrangements; combinations count unordered selections. The fundamental counting principle (multiply the number of choices at each stage) underlies both.
Watch carefully for restrictions, repetition and whether order matters.
Key Points & Formulas
- nPr = n!/(n−r)! (arrangements of r out of n).
- nCr = n!/[r!(n−r)!] (selections of r out of n).
- nCr = nC(n−r); nC0 = nCn = 1.
- Arrangements of n objects with repeats p,q,…: n!/(p! q! …).
Worked Example
Q. In how many ways can a committee of 3 men and 2 women be chosen from 6 men and 4 women?
Choose 3 men: 6C3 = 20. Choose 2 women: 4C2 = 6.
By the counting principle, total = 20 × 6 = 120.