Short division and multiplying by multiples of ten

Short division, digit by digit

$856\div4$: take the digits from the left. $8\div4=2$; $5\div4=1$ remainder $1$, so carry the $1$ to make $16$; $16\div4=4$. Answer $214$. Writing the carried digit small in front of the next digit is what keeps the method reliable.

Multiplying by $10$, $100$, $20$, $300$

$\times10$ moves every digit one place to the left; $\times100$ moves two. For $\times20$, do $\times2$ then $\times10$. For $\times300$, do $\times3$ then $\times100$. Splitting into an easy fact plus a place-value shift beats memorising anything.

Checking with the inverse

Division is checked by multiplying back: $214\times4$ should give $856$. This one line catches almost every slip.