Worked example: a primary bar model drawn on a shared whiteboard
3/5 of a ribbon is 24 cm long. How long is the whole ribbon? On a shared whiteboard the tutor and child draw the same bar, so the tutor sees how the child splits it.
- Step 1Draw the whole ribbonOne bar cut into 5 equal units, because the fraction is in fifths.
- Step 2Shade what you knowShade 3 units and label them 24 cm.
- Step 3Find 1 unit3 units = 24 cm, so 1 unit = 24 ÷ 3 = 8 cm.
- Answer40 cmThe whole ribbon is 5 units. 5 × 8 = 40 cm. Check: 3/5 of 40 cm is 24 cm.
Mistake 1: working out 3/5 of 24 cm. The 24 cm is already the 3 parts, not the whole.
Mistake 2: stopping at 8 cm. The question asks for the whole ribbon.
Try one yourself: 2/7 of a class are boys. There are 10 boys. How many children are in the class?
Answer: 2 units = 10, so 1 unit = 5 and 7 units = 35 children.


