Calculating with decimals, and rounding for accuracy

Multiplying decimals by whole numbers

Ignore the point, multiply, then put it back. $3{,}4\times6$: do $34\times6=204$, and since $3{,}4$ has one decimal place the answer has one too: $20{,}4$. Counting decimal places is a rule you can check, unlike guessing where the point “looks right”.

Dividing decimals by whole numbers

$7{,}2\div4$: divide as usual and keep the point in line: $1{,}8$. Estimating first ($7\div4$ is under $2$) tells you the answer must start with a $1$.

Rounding

To round to one decimal place, look at the next digit only: $5$ or more rounds up. $2{,}349$ to one decimal place is $2{,}3$, because the next digit is $4$ — the $9$ further along does not get a vote.

Round at the end, not the middle

Rounding halfway through a calculation makes the final answer wrong. Keep the full number until the last line.