The idea in brief. Calculate in a common unit, then express the result in the unit the question requests. Put quantities in the same unit before calculating, then return to the unit requested. In the lesson. In the last lesson, you changed measurements in two units into the smaller unit, and back again. Today, we will change to one unit before we calculate, then give the unit the question asks for. Change to the same unit before you add or subtract. Then give the answer in the unit the question asks for.
Lesson video, with the working
Adding and subtracting in two units
5 min 24 s
The film teaches 3 questions in turn. Each question shows first; its working is written as the film explains it.
How do you solve length, mass and volume word problems in P3?
This page teaches word problems where measurements must be put into the same unit before adding or subtracting, and the answer is then given in the unit the question asks for.
What your child learns here
This page teaches word problems where measurements must be put into the same unit before adding or subtracting, and the answer is then given in the unit the question asks for. It applies the P3 outcomes on compound units and on converting between them to one-step and two-step problems.
Change both measurements to the same unit, usually the smaller one, before calculating.
Add or subtract in that one unit, such as millilitres or grams.
Read the question again and give the answer in the unit it asks for, in one unit or two.
Solve two-step problems, for example finding the total of several equal lengths before subtracting it from a roll.
Work backwards: the starting amount is the part used plus the part left.
Does the answer make sense?
Your child's answer must come in the unit the question names, and in a two-unit answer the smaller part stays under one big unit. What remains after cutting or using must be less than what was there, and an amount at first must be more than what is left.
How to practise these questions
The Basic questions practise converting to one unit before adding or subtracting. The Intermediate questions add multi-step stories with length, mass or volume. The Challenge questions ask for the answer in a particular unit, so your child converts at the end as well as the start. When a measurement story stalls, play its solution film and come back to it later from Revision.
Questions parents ask
Why does my child have to change units before adding? Only amounts in the same unit can be added or subtracted directly, in the same way that dollars are added to dollars. Changing both to the smaller unit first keeps the working in one unit throughout.
Are measurement word problems in the Primary 3 syllabus? The Primary 3 syllabus covers measuring in compound units and converting between the larger and smaller unit. The word problems on this page apply those outcomes with adding and subtracting.
Which unit should my child convert to before calculating? Usually the smaller unit, such as grams or millilitres, because then there are only whole numbers to work with. After calculating, your child converts back if the question asks for two units.
