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 be such a number. Since is divisible by , we have . Since is divisible by , we have . Since is divisible by and by , the number is divisible by and . So and with integer and . Therefore , whence .
If , then , which gives 9 possible numbers: 1100, 2200, 3300, 4400, 5500, 6600, 7700, 8800, 9900.
If , then , which gives 4 possibilities: 1200, 2400, 3600, 4800.
If , then , which gives 1 number: 1500.
The case is impossible, since and must be one-digit numbers.
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