Find the 10-digit integer containing all the numbers from 0 to 9 in such a manner that the first -digit number is divisible by .
For example: has 6 digits so it needs to be divisible by 6.
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Here is an outline of a solution but in order to keep it reasonably brief I will leave some of the case checking to the reader:
1 0 ∣ a b c d e f g h i j ⇒ 1 0 ∣ j ⇒ j = 0 5 ∣ a b c d e ⇒ 5 ∣ e ⇒ e = 5 As e = 0
Note due to the divisibility by 2 , 4 , 6 , 8 we have that b , d , f , h are all even so a , c , g , i are all odd.
4 ∣ a b c d ⇒ 4 ∣ c d
A bit of case checking shows d = 2 o r 6 as c is odd.
6 ∣ a b c d 5 f and 3 ∣ a b c ⇒ 3 ∣ d 5 f
Checking cases for d , f gives d = 2 , f = 8 o r d = 6 , f = 4 .
Case 1: d = 2 , f = 8
8 ∣ a b c d 5 8 g h ⇒ 8 ∣ 8 g h ⇒ 8 ∣ g h
As g is odd and h is even checking cases we get g = 1 , 5 , 9 and h = 6 using numbers we have already assigned to eliminate.
6 ∣ a b c 2 5 8 and 9 ∣ a b c 2 5 8 g 6 i ⇒ 3 ∣ g 6 i ⇒ 3 ∣ ( g + i )
Case checking again shows g = 9 , i = 3 is the only possible solution. We also have b = 4 as it is the only remaining even number.
7 ∣ a 4 c 2 5 8 9
Checking the two combinations of 1 , 7 for a , c we see this yields no solutions.
Case 2: d = 6 , f = 4
8 ∣ 4 g h ⇒ 8 ∣ g h
Checking cases gives h = 2 , g = 3 , 7 :
Case 2.1 g = 3
3 ∣ 3 2 i ⇒ i = 1 ( m o d 3 )
As i is odd we have two solutions i = 1 , 7 checking the resulting combinations of a , c for divisibility by 7 shows there are no solutions.
Case 2.2 g = 7
3 ∣ 7 2 i ⇒ i = 0 ( m o d 3 )
As i is odd we get i = 3 , 9 checking resulting combinations of a , c as above shows i = 9 , a = 3 , c = 1 is the only working triplet so our solution is:
3 8 1 6 5 4 7 2 9 0
Note: I have omitted a lot of case checking in this solution purely to try to keep it to a manageable size (it's already too long).