Restricted divisiblity

Number Theory Level pending

Find the number of 4 4 -digit numbers containing non-zero digits such that the number is divisible by 4 4 but not by 8 8 .


The answer is 729.

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1 solution

Eddie The Head
Apr 25, 2014

Let us first make an observation.

If we take any 4 4 consecutive even numbers(say 8 k , 8 k + 2 , 8 k + 4 , 8 k + 6 8k,8k+2,8k+4,8k+6 )then clearly only one among them will be divisible by 4 and not by 8 8 ,ie,of the form 8 k + 4 8k+4 .

Since here we consider only the numbers having 4 4 digits,let the numbers be

1000 a + 100 b + 10 c + 2 1000a+100b+10c+2 1000 a + 100 b + 10 c + 4 1000a+100b+10c+4 1000 a + 100 b + 10 c + 6 1000a+100b+10c+6 1000 a + 100 b + 10 c + 8 1000a+100b+10c+8

So it is quite intuitive that we can divide the set of even numbers into subsets of cardinality 4 4 such that each set contains only one number which is divisible by 4 4 but not by 8 8 .Hence there is a bijection between the number of permissible integers and the number of such 4 4 element subsets.Now for each choice of a , b , c , d a,b,c,d we get a new 4 4 element permissible subset of even numbers and hence the number of possible subsets is equal to the number of such 4 digit integers.

Since a , b , c , d ϵ { 1 , 2 , 3 , 4 , 5 , 6 , 7 , 8 , 9 } a,b,c,d \epsilon \{1,2,3,4,5,6,7,8,9\} ,there are 9 choices for each of a , b , c , d a,b,c,d .Hence the total number of such integers is 9 9 9 = 729 9*9*9 = 729 .

Hence our answer is 729 \boxed{729} .

Note: \textbf{Note:} The cardinality of a set denotes the number of elements in the set.

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