KS2 SATs 2022 — Mathematics Papers 2 & 3: Reasoning

The idea

Measures problems become much easier once everything is in the same unit. The metric conversions to know by heart are: 1 kilometre = 1,000 metres, 1 metre = 100 centimetres, 1 litre = 1,000 millilitres, and 1 kilogram = 1,000 grams.

To change a bigger unit into a smaller one, multiply — kilometres to metres means multiplying by 1,000. To change a smaller unit into a bigger one, divide. A decimal like 1.75 litres becomes 1,750 millilitres, because each litre is worth 1,000 millilitres.

Decide your unit before you calculate, convert every measurement into it, and only then add, subtract or compare. Write the unit in your answer, because a number without its unit answers only half the question.

Worked example

A jug contains 1.75 litres of water. Priya pours out 450 ml. How many millilitres of water are left in the jug?

1. Convert the jug's contents into millilitres: 1.75 × 1,000 = 1,750 ml.
2. Now both amounts are in millilitres, so subtract: 1,750 − 450 = 1,300.

Answer: 1,300 ml

Watch out

• Subtracting 450 from 1.75 without converting the units first.
• Multiplying by 100 instead of 1,000 when changing litres to millilitres.