What does $a\equiv b\pmod n$ mean? What does the $\equiv$ and $b\pmod n$ mean?
for example, what does the following equation mean?
$5x \equiv 7\pmod {24}$? Tomorrow I have a final exam so I really have to know what is it.

Another way of thinking: $a\equiv b\pmod n$ means that the remainder of $a$ divided by $n$ is equal to the remainder of $b$ divided by $n$.
