Question

how does we are know that this number is divisible by 7​

Answer 1

$\huge\underline{\mathbb\red{❥︎A}\green{N}\mathbb\blue{S}\purple{W}\mathbb\orange{E}\pink{R}}\:$

Divisibility by 7: The absolute difference between twice the units digit and the number formed by the rest of the digits must be divisible by 7 7 7 (this process can be repeated for many times until we arrive at a sufficiently small number).

$\textit{Hope this helps you}$