Find the number of -digit numbers containing non-zero digits such that the number is divisible by but not by .
This section requires Javascript.
You are seeing this because something didn't load right. We suggest you, (a) try
refreshing the page, (b) enabling javascript if it is disabled on your browser and,
finally, (c)
loading the
non-javascript version of this page
. We're sorry about the hassle.
Let us first make an observation.
If we take any 4 consecutive even numbers(say 8 k , 8 k + 2 , 8 k + 4 , 8 k + 6 )then clearly only one among them will be divisible by 4 and not by 8 ,ie,of the form 8 k + 4 .
Since here we consider only the numbers having 4 digits,let the numbers be
1 0 0 0 a + 1 0 0 b + 1 0 c + 2 1 0 0 0 a + 1 0 0 b + 1 0 c + 4 1 0 0 0 a + 1 0 0 b + 1 0 c + 6 1 0 0 0 a + 1 0 0 b + 1 0 c + 8
So it is quite intuitive that we can divide the set of even numbers into subsets of cardinality 4 such that each set contains only one number which is divisible by 4 but not by 8 .Hence there is a bijection between the number of permissible integers and the number of such 4 element subsets.Now for each choice of a , b , c , d we get a new 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 } ,there are 9 choices for each of a , b , c , d .Hence the total number of such integers is 9 ∗ 9 ∗ 9 = 7 2 9 .
Hence our answer is 7 2 9 .
Note: The cardinality of a set denotes the number of elements in the set.