Common Questions on Permutations and Combinations with Answers
FAQs on Permutations and Combinations: Practice Paper for Students
1. What is the difference between permutations and combinations?
Permutations are arrangements where order matters, while combinations refer to selections where order does not matter.
Key Differences:
- Permutations: Order matters (P(n, r) = n! / (n-r)!).
- Combinations: Order does not matter (C(n, r) = n! / [r! (n-r)!]).
- Permutations count arrangements, combinations count groups or choices.
2. How do you calculate the number of permutations of n objects taken r at a time?
The number of permutations of n objects taken r at a time is P(n, r) = n! / (n-r)!.
3. What is the formula for combinations and when is it used?
The formula for combinations is C(n, r) = n! / [r!(n - r)!], and it is used when the order of selection does not matter.
Use combinations when:
- Choosing teams or groups.
- Selecting items without concern for order.
4. In how many ways can a committee of 3 be selected from a group of 7 people?
A committee of 3 from 7 people can be selected in C(7, 3) = 35 ways, using the combinations formula.
- n = 7 (people), r = 3 (selected)
- Calculation: 7! / (3! 4!) = 35 ways
5. What is the value of 0 factorial (0!) and why?
0! is defined as 1 by convention to make permutation and combination formulas consistent.
- This ensures factorial expressions work properly for calculations involving zero selections.
6. How do you solve permutation problems with repeated objects?
For permutations with repeated objects, use n! / (p1! p2! ... pk!) where n = total objects and p1, p2, etc. are counts of repeats.
Steps:
- Count total items (n).
- Find how many times each item repeats (p1, p2,...).
- Apply the formula above.
7. Can you give examples where permutations are used in real life?
Permutations are used where arrangement or order is important.
- Arranging books on a shelf (order matters).
- Assigning seats to people.
- Forming different number codes or passwords.
8. How many 3-digit numbers can be formed using the digits 1, 2, 3, 4 without repetition?
There are P(4, 3) = 24 ways to form 3-digit numbers from 1, 2, 3, 4, without repeating digits.
- First digit: 4 choices
- Second digit: 3 choices
- Third digit: 2 choices
- Total: 4 × 3 × 2 = 24
9. What is the significance of nCr in combinations?
nCr represents the number of ways to select r items from n without considering order, key in combination calculations.
- n = total items
- r = items chosen
- Used in counting possible groups, teams and selections
10. What are the main applications of permutations and combinations in exams?
Permutations and combinations are applied in probability, arrangement problems, and selection scenarios in maths and competitive exams.
- Probability calculation
- Counting arrangements and groupings
- Questions on selection of students, committees, or digits/letters
- Logical reasoning and MCQ pattern questions






















