Maths-
General
Easy

Question

How many different nine digit numbers can be formed from the number 2,2,3,3,5,5,8,8,8 by rearranging its digits so that the odd digits occupy even position ?

  1. 16    
  2. 36    
  3. 60    
  4. 180    

hintHint:

 Here we need to find the total number of nine digit numbers that can be formed using the given digits. We will count the number of even places present for the odd digits and then we will find the number of odd places present for the even digits. Then we will find the number of ways to arrange the odd digits and then we will find the number of ways to arrange the even digits and to get the final answer, we will multiply both of them.

The correct answer is: 60


    Detailed Solution
    Here we need to find the total number of nine digit numbers that can be formed using the given digits i.e. 2, 2, 3, 3, 5, 5, 8, 8, 8.
    X minus X minus X minus X minus X
    H e r e comma space s y m b o l space left parenthesis negative right parenthesis space space i s space f o r space t h e space e v e n space p l a c e s space a n d space left parenthesis X right parenthesis space space i s space f o r space t h e space o d d space p l a c e s space o f space t h e space d i g i t space n u m b e r.
    The digits which are even are 2, 2, 8, 8 and 8.
    N u m b e r space o f space e v e n space d i g i t s space equals 5
    The digits which are odd are 3, 3, 5 and 5.

    N u m b e r space o f space o d d space d i g i t s space equals 4
    We have to arrange the odd digits in even places.
    N u m b e r space o f space w a y s space t o space a r r a n g e space t h e space o d d space d i g i t s space i n space 4 space e v e n space p l a c e s space equals fraction numerator space 4 factorial over denominator 2 factorial cross times 2 factorial space end fraction
    On finding the value of the factorials, we get

    N u m b e r space o f space w a y s space t o space a r r a n g e space t h e space o d d space d i g i t s space i n space 4 space e v e n space p l a c e s space equals space fraction numerator 4 cross times 3 cross times 2 cross times 1 over denominator 2 cross times 1 cross times 2 cross times 1 end fraction
    On further simplification, we get
    N u m b e r space o f space w a y s space t o space a r r a n g e space t h e space o d d space d i g i t s space i n space 4 space e v e n space p l a c e s space equals 6
    Now, we have to arrange the even digits in odd places.

    N u m b e r space o f space w a y s space t o space a r r a n g e space t h e space e v e n space d i g i t s space i n space 5 space o d d space p l a c e s space equals fraction numerator 5 factorial over denominator 2 factorial cross times 3 factorial end fraction
    On finding the value of the factorials, we get
    N u m b e r space o f space w a y s space t o space a r r a n g e space t h e space e v e n space d i g i t s space i n space 5 space o d d space p l a c e s space equals fraction numerator 5 cross times 4 cross times 3 cross times 2 cross times 1 over denominator 2 cross times 1 cross times 3 cross times 2 cross times 1 end fraction
    On further simplification, we get

    N u m b e r space o f space w a y s space t o space a r r a n g e space t h e space e v e n space d i g i t s space i n space 5 space o d d space p l a c e s space equals 10
    T o t a l space n u m b e r space o f space 9 space d i g i t s space n u m b e r space equals space 6 cross times 10 space equals space 60
    H e n c e comma space t h e space r e q u i r e d space n u m b e r space o f space 9 space d i g i t space n u m b e r s space equals 60

    Here we have obtained the total number of 9 digit numbers using the given digits. While finding the number of ways to arrange the odd digits in 5 even places, we have divided the 4! by 2because the digit 3 were occurring two times and the digit 5 were occurring 2 times. Here we can make a mistake by conserving the number of even digits 4 and the number of odd digits 5, which will result in the wrong answer.

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