���˿���

So, the number \(5.743\) is said to have 3 decimal places. The number \(5.1492\) has four decimal places, while \(4.34\) has two decimal places.

Key point

To round a number to a given number of decimal places, look at the number in the next decimal place. If it's less than 5, round down. If it's 5 or more, round up.

Example

Round \(9.6371\) to \(2\) decimal places
This means we need \(2\) digits after the decimal point.

Since the next digit \(7\), is more than \(5\), we round the \(3\) up.
\(9.6371 = 9.64\) (\(2\) decimal places)

Key point

To round a number to a given number of decimal places, the answer must have that number of decimal places - even if you have to add some zeros.

For example, rounding \(3.40021\) to two decimal places gives \(3.40\).

You need to write both decimal places, even though the second number is a zero, to show that you rounded to two decimal places.

Remember to look at the number after the one you're interested in. If it's less than \(5\), round down. If it's \(5\) or more, round up.

Another way of rounding numbers is to count only the first few digits (maybe \(1\), \(2\) or \(3\) figures) that have a value attached to them.

This method of rounding is called significant figures and it’s often used with larger numbers, or very small numbers.

Rounding \(12.756\) or \(4.543\) to one decimal place seems sensible, as the rounded figures are roughly equal to the actual value.

\(12.756 = 12.8\) (\(1\) decimal place)

\(4.543 = 4.5\) (\(1\) decimal place)

But what happens if you round a very small number to one decimal place?

\(0.00546 = 0.0\) (\(1\) decimal place)
\(0.00213 = 0.0\) (\(1\) decimal place)

This is not a useful answer.

Another way to find an approximate answer with very small numbers is to use significant figures.