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jmc

number theory senior

Problem

What is the smallest positive five-digit integer, with all different digits, that is divisible by each of its non-zero digits? Note that one of the digits of the original integer may be a zero.
Solution
We know that the five-digit number must be divisible by each of its non-zero digits. We should include the digit zero since it does not have to be included as a possible divisor and it will keep the integer smaller. Knowing this, the least possible five-digit number that we could try is . Any number we choose will be divisible by one. We also see that it is even, and therefore, divisible by two. However, the two-digit number formed by its last two digit () is not divisible by four, and therefore, neither is the five-digit number. We also see that the sum of the five digits is , and since is not divisible by three, neither is the five-digit number. However, notice that by increasing the five-digit number by two to form the number , we create another even number and increase the digit-sum to (which takes care of the number being divisible by three). We have now eliminated the digit of four and added the digit of six, which is fine because being divisible by both two and three means that it is divisible by six. Our five-digit number is .
Final answer
10,\!236