Mei Ling has a roll of ribbon 4 m long. She cuts off 8 equal pieces, each 35 cm long. What is the length of the ribbon left on the roll? Give your answer in metres.
Length of ribbon left = 4 − 2.8 = 1.2 m
Ans: 1.2 m
How to solve it
Find how much came off: 8 pieces of 35 centimetres is 8 times 35, which is 280 centimetres.
The roll is in metres and the cut is in centimetres, so they cannot be subtracted yet.
Change to the same unit. There are 100 centimetres in a metre, so 280 divided by 100 is 2.8 metres.
Now subtract in metres: 4 minus 2.8 is 1.2 metres of ribbon left.
One strip, and two units written on it. Metres above, centimetres below. The middle line is the one that makes them the same. So change the units first, and then subtract.
Practice questions with worked solutions
Question 1 is free to try. The other 28 open in the free trial, each with its full working.
Question 1 of 29 · Basic · free to tryPriya's school bag has a mass of 2.7 kg. She puts in a book with a mass of…Priya's school bag has a mass of…
Priya's school bag has a mass of 2.7 kg. She puts in a book with a mass of 850 g. What is the total mass of the bag now? Give your answer in kilograms.
book mass in kilograms = converted value of 850 g = 850 ÷ 1000 = 0.85 kg
total mass of the bag = 2.7 + 0.85 = 3.55 kg
Answer: 3.55 kg
For parents
How do you solve P5 mixed units word problems?
Here the unit conversions of Primary 5 decimals meet a word problem: two quantities written in different units are brought to one unit before they are added, subtracted, compared or shared.
What your child learns here
Here the unit conversions of Primary 5 decimals meet a word problem: two quantities written in different units are brought to one unit before they are added, subtracted, compared or shared. It applies the syllabus outcome on converting measurements to multi-step questions.
Spot when two quantities in one question are measured in different units.
Convert one of them so both use the same unit, moving the decimal point by the right number of places.
Carry out the operation, then check the answer is in the unit the answer line asks for.
Use a size check: adding a small mass to a larger one cannot leave the total almost unchanged.
Does the answer make sense?
When two amounts are put together, the answer must be larger than the bigger one; when some is used, cut off or poured out, what is left must be less than the starting amount. If your child writes both amounts in grams, metres or millilitres first, the comparison is easy to see.
How to practise these questions
Basic questions train adding, comparing and subtracting measures given in two units, such as kilograms with grams or litres with millilitres, and giving the answer in the unit asked. Intermediate questions chain two or three steps, so a converted amount is shared, repeated or set against a larger total. The Challenge questions scale or recover amounts before converting, and only the last line settles which unit the answer must be in. If a unit question goes astray, watch its solution film, then come back to it from Revision another day.
Questions parents ask
Should my child convert to the bigger unit or the smaller unit? Either works, as long as both quantities are in the same unit before calculating. Converting to the unit the answer is wanted in saves a step at the end.
Are two-unit word problems in the Primary 5 syllabus? The syllabus outcome is converting a measurement from a smaller unit to a larger unit in decimal form, and back. This page applies it to word problems where two units appear together.
Can my child use a calculator for unit conversions? The related Primary 5 outcome on multiplying and dividing decimals by 10, 100 and 1000 is listed as without calculator, so it is worth practising by hand.
