How many 4-digit positive integers which comprise of at most 2 distinct non-zero digits, are divisible by 11?
Note/Hint: A number can be in the form of a a a a or b b b b .
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For those interested, the numbers are as follows:
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This leaves out numbers with zeros in them.
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Thanks. I removed "non-zero" when I edited the problem for clarity.
Those who answered 171 have been marked correct.
This solution shows that these 2 groups satisfy the conditions in the problem, meaning that this condition is sufficient.
However, it does not show that these 2 groups are the only 2 groups, meaning that we haven't shown this condition is necessary. We need to explain why numbers of the form a a a b , a a b a etc cannot be multiples of 11, for any values of a and b .
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There are two groups of four-digit numbers divisible by 1 1 that fulfill the conditions of values of a and b . The first group is consisted of palindromes. Palindromes are numbers that are read the same from left to right and from right to left. Here is a proof of this claim, taking for example a number with a form of a b b a
a b b a = 1 0 0 0 × a + 1 0 0 × b + 1 0 × b + a = 1 0 0 1 a + 1 1 0 b
Because 1 1 ∣ 1 0 0 1 and 1 1 ∣ 1 1 0 = > 1 1 ∣ a b b a
Palindromes come in the following forms:
The other group consists of numbers that are in forms of:
And here is the proof for this claim, taking for example a number with a form of a a b b :
a a b b = 1 0 0 0 × a + 1 0 0 × a + 1 0 × b + b = 1 1 0 0 a + 1 1 b
Because 1 1 ∣ 1 1 0 0 and 1 1 ∣ 1 1 = > 1 1 ∣ a a b b
There is a total of 1 5 3 numbers that fulfill all of the requirements above.