Question 158
Let $k$ be a positive integer. A positive integer $n$ is said to be a $k$-flip if the digits of $n$ are reversed in order when it is multiplied by $k$. For example, $1089$ is a $9$-flip because $1089 \times 9 = 9801$, and $21978$ is a $4$-flip because $21978 \times 4 = 87912$. Explain why there is no $7$-flip integer.