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.

Let $a=qn+r_{1}$ and $b=pn+r_{2}$, where $0\leq r_{1},r_{2}<n$. Then $$r_{1}=r_{2}.$$ 
$r_{1}$ and $r_{2}$ are remainders when $a$ and $b$ are divided by $n$. 
