Question

# The number lock of a suitcase has 4 wheels each labelled with ten digits i.e. From $0$ to 9. The lock opens with a sequence of four digits with no repeats. What is the probability of a person getting the right sequence to open the suitcase?

Hint: In order to solve such type of question firstly there are number of wheels in number lock of suitcase$= 4$.As per given statement there is no repetition of digit between From $0$ to $9$ i.e. first digit can be between $0$ to $9$.Thus no of digit $= 10$.

Now, the first wheel can have any one of the tens digits from $0$ to $9$. So, a wheel can have any of the $10$ digits.
Since, repetition is not allowed, so the second wheel can have any of the remaining $9$ digits.
And the fourth wheel can have any of the remaining $7$ digits.
So, number of four digit lock code that can be formed without repetition of digit$= 10 \times 9 \times 8 \times 7 = 5040$
So, a total four digit number formed$= 5040$ .
Hence, required probability$= \dfrac{1}{{5040}}$.
Note: Whenever we face such a type of question the key concept is that. Since there is no repetition one digit is used cannot be repeated again and as per there are a number of wheels in the number lock of suitcase$= 4$ there are just 4 digits and cannot be exceeded.