I am trying to prove:
$a\mid b \land b\mid c \Rightarrow a\mid c $
$a\mid b$ means that a divides b if there is an integer k, that $b=k\cdot a$
Please give me a hint on how to start, because I have no idea.
If $a | b$ and $b | c$, you have $b = ka$ and $c = jb$. Thus $c = (jk)a$. Hence $a | c$.