Proving $d\ell= ad+b$ for integers that $d$ must divide $b$.
If $d\ell$ is a product we can write it as $\underbrace{d+d+d+\ldots}_{\ell \text{ times}}$ and $ad$ as a sum of $\underbrace{d+d+d+\ldots}_{a \text{ times}}$ which means $b$ must be a sum of $d$'s, but how can I prove that to be true?