がい数（四捨五入）と見積もり

To round to the nearest 10, look at the ones digit. If it is 0, 1, 2, 3 or 4 you round down; if it is 5, 6, 7, 8 or 9 you round up. The easy way to remember it: 5 or more rounds up. For example, $283$ has a ones digit of $3$, so it becomes $280$; $67$ has a ones digit of $7$, so it becomes $70$.

To round to the nearest 100, look at the tens digit. If it is 5 or more you round up; if it is 4 or less you round down. For example, $638$ has a tens digit of $3$, so it becomes $600$; $472$ has a tens digit of $7$, so it becomes $500$. When the tens digit is exactly $5$, like in $350$, the rule still says round up, giving $400$.

Before you calculate, you can round each number first to see roughly how big the answer should be. This is called estimating. For example, for $312+489$ you can do $300+500=800$, so the answer is about $800$. Estimating is a great way to check that your real answer looks sensible.