Master Counting Principles with Easy Tricks and Examples

Practice Questions

Engineering Mathematics, Counting Principle, Practice Questions

Question 1: Forming Words with Vowels

Question:
How many words (with or without meaning) can be formed using vowels? (Repetition allowed and not allowed)

  • Vowels are: A, E, I, O, U β†’ 5 vowels.

Answer:

  • If repetition is allowed:
    Each position can have any vowel.
    So,

    5Γ—5Γ—5=53=125 words

  • If repetition is not allowed:
    Each position must have a different vowel.
    So,

    5Γ—4Γ—3=60Β words

Explanation (Simple Language):

  • Since all letters are used together, it’s a case of multiplication.

  • With repetition β†’ Same letter can appear again.

  • Without repetition β†’ No letter can be repeated; choices decrease.


Question 2: Posting Letters into Letterboxes

Question:
There are 4 letters and 5 letterboxes. In how many ways can the letters be posted?

Answer:

  • Repetition is allowed β€” because a letterbox can receive multiple letters.
    (No rule that one box can hold only one letter.)

  • So,

    5Γ—5Γ—5Γ—5=54 =625 ways

Short Approach Explanation:

  • Letters are moving (not letterboxes).

  • Make blanks for moving items β†’ (Letter1, Letter2, Letter3, Letter4).

  • Fill each blank with number of options (letterboxes = 5).

⚑ Note:
Answer is not 45Β because letterboxes cannot move toward the letters β€” that would be weird!


Question 3: Wearing Rings on Fingers

Question:
There are 3 rings and 4 fingers. In how many ways can the rings be worn?

Answer:

  • No restriction (repetition allowed):
    Rings are moving to fingers.

    So,

    4Γ—4Γ—4=43 =64 ways

  • If repetition is not allowed (one finger holds only one ring):

    4Γ—3Γ—2=24 ways

Short Approach Explanation:

  • Rings are movable β†’ make blanks for rings.

  • Fill each blank with number of fingers (4 options initially).

🚫 Wrong Approach:
If you mistakenly do 34, it means fingers are moving to rings β€” not natural during a wedding ceremony (and sounds like a forced marriage πŸ˜…).


Question 4: Seating Persons on Chairs (More Chairs)

Question:
There are 5 persons and 8 chairs. How many seating arrangements are possible?

Answer:

  • 8Γ—7Γ—6Γ—5Γ—4=6720 ways

Question 5: Seating Persons on Chairs (More Persons)

Question:
There are 8 persons and 5 chairs. How many seating arrangements are possible?

Answer:

This question is senseless in basic counting because you can’t seat 8 persons on 5 chairs without extra conditions like:

  • Sitting on laps (which is not usual in standard problems πŸ˜…)

  • Or standing/switching chairs (not considered here)

Thus, invalid without additional context.


Question 6: Password Formation

Question:
How many 3-letter passwords can be formed using 6 different alphabets if repetition is allowed?

Answer:
Each position has 6 choices.

6Γ—6Γ—6=63=216 passwords


Question 7: Team Selection

Question:
A captain wants to select either a bowler (6 choices) or a batsman (5 choices) for a match. How many ways can he select?

Answer:
Feeling of either/or β†’ Addition principle.

6+5=11 ways


Conclusion

Practicing these basic counting problems builds a strong foundation for more advanced topics like permutations, combinations, and probability.
Always remember:

  • Multiplication β†’ Feeling of “and”, “all”, “together”.

  • Addition β†’ Feeling of “or”, “either”, “only one”.

Next Up: We’ll explore permutations in detail with real-world examples!


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