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    

Hint:

 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.
    O space e space O space e space O space e space O space e space O
    Here, e is for the even places and O is for the odd places of the digit number.
    The digits which are even are 2, 2, 8, 8 and 8.

    Number of even digits  5
    The digits which are odd are 3, 3, 5 and 5.
    Number of odd digits 4
    We have to arrange the odd digits in even places.

    Number of ways to arrange the odd digits in 4 even places = fraction numerator 4 factorial over denominator 2 factorial cross times 2 factorial end fraction = 6

    Now, we have to arrange the even digits in odd places.

    Number of ways to arrange the even digits in 5 odd places  = fraction numerator 5 factorial over denominator 2 factorial cross times 3 factorial end fraction space equals space 10

    Total number of 9 digits number = 6 cross times 10 space equals space 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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