Solve problems involving the calculation and conversion of units of measure, using decimal notation up to 3 decimal places where appropriate

What this means, in plain words

A decimal writes parts of a whole using the places after the point: tenths, then hundredths, then thousandths.

Converting means saying the same amount in a different unit, such as 1,500 metres as 1.5 kilometres.

This is problem solving: the maths is met inside a situation, so the first job is working out which operation the question is asking for.