What is the smallest whole number that is equal to seven times the sum of its digits?

Question

Find a four digit number, with four different digits, that equal to the number formed by its digits in descending order minus the number formed by its digits in ascending order.

Answer 1

21

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If the number is xy, it can be written as 10x+y
10x+y=7*(x+y) => 3x=6y =>x=2y
The smallest xy is 21