Answer
Verified
419.7k+ views
Hint: At least one digit repeated numbers can be given by the difference of total numbers of 5 digits from the digits 0, 1, 2, 3, 4, 5, 6, 7, 8, 9. with allowed repetition and the numbers with no repetition (distinct digits). Selection of ‘r’ different things from the total ‘n’ different things is given by $ C_{r}^{n}=\dfrac{n!}{r!\left( n-r \right)!} $ . Arrangement of ‘r’ digit numbers from ‘n’ digits with allowed repetition is given as $ {{n}^{r}} $ .
Complete step-by-step answer:
As we know there are 10 digits 0, 1, 2, 3, 4, 5, 6, 7, 8, 9 that can be used for making a 5 digit telephone number and as we need to determine total telephonic numbers of 5 digits having at least one digit repeated. It means there may be repetition of one digit or repetition of two digits or three but all the digits involved for making 5 digits cannot be distinct to each other i.e. at least one digit should be repeated.
So, in other words we need to calculate all the 5 digit numbers except the numbers which have all 5 digit different. It means total telephonic numbers with 5 digits having at least one digit repeated can be calculated by the difference of total numbers of 5 digits and the numbers with all distinct 5 digits. So, total 5 digit numbers with the help of 10 digit numbers and repetition is allowed with the formation of numbers can be given as
As digits can be repeated it means there are 10 possibilities for a single digit. Hence, total 5 digit numbers can be given as
$ 10\times 10\times 10\times 10\times 10={{\left( 10 \right)}^{5}} $
Now, as we know, selection of r different things from ‘n’ different numbers is given by relation $ C_{r}^{n}=\dfrac{n!}{r!\left( n-r \right)!} $ . Hence, we can select 5 different digits from the total 10 digits i.e. 0, 1, 2, 3, 4, 5, 6, 7, 8, 9 by $ C_{5}^{10} $ ways. Now, we need to arrange these digits to get the total number of 5 digits.
Now, as we know that the total 5 digit numbers with no repetition can be calculated as
The first digit can be filled by 5 ways and second by 4 because the same digit cannot be occupied at two places as repetition is not allowed. Similarly, 3rd place can be filled by 3 ways as two digits are already occupied with two different numbers. Hence, we can get total number of 5 digits telephonic number with 5 selected digits
$ C_{5}^{10}\times 5\times 4\times 3\times 2\times 1=C_{5}^{10}\times 120 $
So, numbers with at least one digit repeated are given by the difference of total numbers possible with repetition and without repetition. So, we get total 5 digit telephone numbers with having at least one of the digit repeated
$ {{\left( 10 \right)}^{5}}-C_{5}^{10}\times 5\times 4\times 3\times 2\times 1 $
= 100,000 – 30240
= 69760.
Hence, option (d) is correct.
Note: One may go wrong if he or she select 5 digits from the given numbers by $ C_{5}^{10} $ ways for the formation of numbers with repetition allowed case as $ C_{5}^{10} $ used for selecting 5 different numbers from 10 different numbers. So, don’t be confused with that part. One may use formula $ P_{r}^{n} $ for selecting 5 different digits and arranging them for the formation of numbers with distinct digits. As $ P_{r}^{n} $ gives selection and arrangement both at a time. So, it can be another approach for this step in the solution.
One may arrange the 5 different digits by 5! Ways directly for the second case in the solution. The arrangement of ‘n’ different things can be given by formula n!.
Complete step-by-step answer:
As we know there are 10 digits 0, 1, 2, 3, 4, 5, 6, 7, 8, 9 that can be used for making a 5 digit telephone number and as we need to determine total telephonic numbers of 5 digits having at least one digit repeated. It means there may be repetition of one digit or repetition of two digits or three but all the digits involved for making 5 digits cannot be distinct to each other i.e. at least one digit should be repeated.
So, in other words we need to calculate all the 5 digit numbers except the numbers which have all 5 digit different. It means total telephonic numbers with 5 digits having at least one digit repeated can be calculated by the difference of total numbers of 5 digits and the numbers with all distinct 5 digits. So, total 5 digit numbers with the help of 10 digit numbers and repetition is allowed with the formation of numbers can be given as
As digits can be repeated it means there are 10 possibilities for a single digit. Hence, total 5 digit numbers can be given as
$ 10\times 10\times 10\times 10\times 10={{\left( 10 \right)}^{5}} $
Now, as we know, selection of r different things from ‘n’ different numbers is given by relation $ C_{r}^{n}=\dfrac{n!}{r!\left( n-r \right)!} $ . Hence, we can select 5 different digits from the total 10 digits i.e. 0, 1, 2, 3, 4, 5, 6, 7, 8, 9 by $ C_{5}^{10} $ ways. Now, we need to arrange these digits to get the total number of 5 digits.
Now, as we know that the total 5 digit numbers with no repetition can be calculated as
The first digit can be filled by 5 ways and second by 4 because the same digit cannot be occupied at two places as repetition is not allowed. Similarly, 3rd place can be filled by 3 ways as two digits are already occupied with two different numbers. Hence, we can get total number of 5 digits telephonic number with 5 selected digits
$ C_{5}^{10}\times 5\times 4\times 3\times 2\times 1=C_{5}^{10}\times 120 $
So, numbers with at least one digit repeated are given by the difference of total numbers possible with repetition and without repetition. So, we get total 5 digit telephone numbers with having at least one of the digit repeated
$ {{\left( 10 \right)}^{5}}-C_{5}^{10}\times 5\times 4\times 3\times 2\times 1 $
= 100,000 – 30240
= 69760.
Hence, option (d) is correct.
Note: One may go wrong if he or she select 5 digits from the given numbers by $ C_{5}^{10} $ ways for the formation of numbers with repetition allowed case as $ C_{5}^{10} $ used for selecting 5 different numbers from 10 different numbers. So, don’t be confused with that part. One may use formula $ P_{r}^{n} $ for selecting 5 different digits and arranging them for the formation of numbers with distinct digits. As $ P_{r}^{n} $ gives selection and arrangement both at a time. So, it can be another approach for this step in the solution.
One may arrange the 5 different digits by 5! Ways directly for the second case in the solution. The arrangement of ‘n’ different things can be given by formula n!.
Recently Updated Pages
Basicity of sulphurous acid and sulphuric acid are
Assertion The resistivity of a semiconductor increases class 13 physics CBSE
The Equation xxx + 2 is Satisfied when x is Equal to Class 10 Maths
What is the stopping potential when the metal with class 12 physics JEE_Main
The momentum of a photon is 2 times 10 16gm cmsec Its class 12 physics JEE_Main
Using the following information to help you answer class 12 chemistry CBSE
Trending doubts
Difference Between Plant Cell and Animal Cell
Difference between Prokaryotic cell and Eukaryotic class 11 biology CBSE
Fill the blanks with the suitable prepositions 1 The class 9 english CBSE
Change the following sentences into negative and interrogative class 10 english CBSE
Summary of the poem Where the Mind is Without Fear class 8 english CBSE
Give 10 examples for herbs , shrubs , climbers , creepers
Write an application to the principal requesting five class 10 english CBSE
What organs are located on the left side of your body class 11 biology CBSE
What is the z value for a 90 95 and 99 percent confidence class 11 maths CBSE