Maths Olympiad Prep

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Number theory Difficulty 5.3 AIME, harder Prove it Estonia

Find all four-digit numbers, which after deleting any one digit turn into a three-digit number that is a divisor of the original number.

Solution

Let abcd\overline{abcd} be such a number. Since abcd\overline{abcd} is divisible by abc\overline{abc}, we have d=0d = 0. Since abcd=abc0\overline{abcd} = \overline{abc0} is divisible by abd=ab0\overline{abd} = \overline{ab0}, we have c=0c = 0. Since abcd=ab00\overline{abcd} = \overline{ab00} is divisible by acd=a00\overline{acd} = \overline{a00} and by bcd=b00\overline{bcd} = \overline{b00}, the number ab\overline{ab} is divisible by aa and bb. So b=axb = ax and 10a=by10a = by with integer xx and yy. Therefore 10a=axy10a = axy, whence xy=10xy = 10.

If x=1,y=10x = 1, y = 10, then a=ba = b, which gives 9 possible numbers: 1100, 2200, 3300, 4400, 5500, 6600, 7700, 8800, 9900.

If x=2,y=5x = 2, y = 5, then 2a=b2a = b, which gives 4 possibilities: 1200, 2400, 3600, 4800.

If x=5,y=2x = 5, y = 2, then 5a=b5a = b, which gives 1 number: 1500.

The case x=10,y=1x = 10, y = 1 is impossible, since aa and bb must be one-digit numbers.

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Source: MathNet, licensed CC-BY-4.0. Statement reproduced verbatim; metadata (topic, difficulty) added by this project.