Commonly-made mistake for Length

Look at the pictures above, which one of the following is the correct way to measure the green rectangle?


That's right! It is the picture below. Any object to be measured must be placed an the '0' cm mark.


Highlight to children that some rulers have measurements of 'cm' and 'inches' on them so they have to identify and use the correct scale.

If the object (yellow rectangle) to be measured is not placed on the '0' cm mark, the children need to know how to find the difference in the length by subtracting the beginning mark from the end mark. For example, the yellow rectangle is 2cm - 1cm = 1cm long.








Referring to the Math Workbook 1A part 2 page 69, questions such as this:




          8cm                                           2cm
(i) The straw is _____ cm longer than the key.


Children are encouraged to write the measurement on top of the objects. A commonly-made mistake is children often write 8cm as the answer because they did not see that this question needs them to compare the lengths of 2 objects. The words in bold 'longer than' is a clue. Writing the measurements above the objects will help them to see this relationship. Finding the difference (how much more or less) means we do subtraction.

Labelling lines

Question: Draw a line BC that is 6cm.


The top picture shows the correct answer. Get your child to explain to you why the bottom 2 pictures show the wrong answers.


Drawing straight lines freehand is unacceptable.




Other learning objectives:

• Use a string to measure lengths of curves
• Solve one- and two-step word problems by relating them with addition and subtraction concepts such as 'part-whole', 'adding-on', 'taking-away' and 'comparing'
• Solve one- and two-step word problems by relating them with multiplication and division concepts such as 'group and item' and 'multiplying'
• draw models to solve the problems

Thank you for your attention!