Question

A number is such that its ten digit is twice its unit digit. Prove that the number itself must be seven times the sum of its digit

Answer 1

x - ten digit
y - unit digit


`x=2y\\ 10x+y=7(x+y)\\\\ 10\cdot2y+y=7(2y+y)\\ 20y+y=14y+7y\\ 21y=21y \Rightarrow y\in \mathb{R}\Rightarrow x\in \mathb{R}`

So it's true.