Test of divisibility for 3



The sum of the digits of any  number that is divisible by '3' is divisible by 3.



For instance, take the  number 54372.

Sum of its digits is 5 + 4 + 3 + 7 + 2 = 21.

As 21 is  divisible by '3', 54372 is also divisible by 3.



There are six digits  viz., 0, 1, 2, 3, 4 and 5. To form 5-digit numbers we need exactly 5 digits. So  we should not be using one of the digits.



The sum of all the six digits  0, 1, 2, 3, 4 and 5 is 15. We know that any number is divisible by 3 if and only  if the sum of its digits are divisible by '3'.



Combining the two criteria  that we use only 5 of the 6 digits and pick them in such a way that the sum is  divisible by 3, we should not use either '0' or '3' while forming the five digit  numbers.



Case 1

If we do not use '0', then the remaining 5 digits can  be arranged in 5! ways = 120 numbers.



Case 2

If we do not use '3',  then the arrangements should take into account that '0' cannot be the first  digit as a 5-digit number will not start with '0'.



. The first digit from  the left can be any of the 4 digits 1, 2, 4 or 5.



Then the remaining 4  digits including '0' can be arranged in the other 4 places in 4!  ways.



So, there will be 4*4! numbers = 4*24 = 96  numbers.



Combining Case 1 and Case 2, there are a total of 120 + 96 = 216  5 digit numbers divisible by '3' that can be formed using the digits 0 to 5.                                                                                                                            


						

							
                                                     
              FOR A NUMBER TO BE DIVISIBLE BY 3 THE SUM OF ITS DIGITS SHOULD BE DIVISIBLE BY 3 . SO, AMONG THE 6 DIGITS , 5 DIGITS
              WHOSE SUM IS DIVISIBLE BY 3 MUST BE SELECTED.
                           0+1+2+3+4+5 = 15 ( a multiple of 3)
               SO THE FIVE DIGITS CAN BE OBTAINED BY ELIMINATING A 3 MULTIPLE . 
                   15 - 3q = 3(5 - q) . SO IT IS DIVISIBLE BY 3.
                    AMONG 0,1,2,3,4,5 ;  0 AND 3 ARE THE MULTIPLES OF 3 .
                   SO, THE 5 DIGITS MUST BE  (1,2,3,4,5) OR (0,1,2,4,5)
                     FIRST LET US TAKE (1,2,3,4,5) . ACCORDING TO PERMUTATION LAW , IT CAN BE ARRANGED IN 5p5 WAYS
                                                                                         5P5  = 5 x 4 x 3 x 2 x 1 = 120
                                         (0,1,2,4,5) . IF ZERO IS THE FIRST DIGIT IT BECOMES A 4 DIGIT NUMBER .      
                   NUMBER OF  NUMBERS HAVING 0 AS FIRST DIGIT ARE                             
                                                  4 x 3 x 2 x 1 = 24                                 
                          SO, NUMBER OF 5 DIGIT NUMBER FORMED FROM (0,1,2,4,5)  ARE 120 - 24 = 96.
                             TOTAL  NUMBER OF 5 DIGIT NUMBER FORMED FROM  (0, 1,2,3,4,5)   ARE 120 + 96 = 216.