This week Ioan and Finny decided to base some of their length and perimeter work on the series of Tin Tin books by Hergé. It’s two years since Ioan first fell in love with these books. It’s a passion that he’s now shared with his brothers, with Finny keen to play the role of Snowy and Cian happy to be Captain Haddock.


  • The Adventures of Tin Tin by Georges Remi, who wrote under the pen name Hergé
  • Whiteboards and pens
  • Pirate ship
  • Ark
  • Magnatiles


The viewfinder wasn’t working on the camera, so I just had to press record, point and hope they were in the frame.

Challenge 1: Captain Haddock knows there is treasure hidden in one of these boats after the fight between Sir Francis Haddock and Red Rackham. It is in the hull of the boat with the largest perimeter.

Finny came up with this first question, he wrote it up on the whiteboard, not yet knowing the answer. While he was writing a second question, I measured the two perimeters and hid the treasure in the hull of the boat with the largest perimeter.

The Unicorn

First Finny needed to work out the length of the four sides of the Unicorn’s hull. Length is the size of an object or distance from one point to the other.

As the boys explained in the video above, perimeter is the distance around a closed 2-D shape. For any shape we can find this by measuring the total length of the lines around the shape. In the example below, the two boats were 3-D, so to measure perimeter they had to find the flat surface on the side of the hull to measure around.

The Jolly Roger

Challenge 2: Thompson and Tomson join some of these equilateral triangles together. Their shapes have the same perimeter of 24cm. What could their shapes be?

Finny’s second question mentioned that the perimeter of the shapes had a perimeter of 24cm. As the length of one side of the triangle was 4cm, Finny worked out that 4 went into 24 six times. Ioan then used that information to explain that meant the shapes would have 6 sides.

Using the equilateral triangles to make some of the 6 sided shapes that Thompson and Thomson could have made.

Challenge 3: King Ottakar’s Sceptre has been stolen. Tin Tin needs to work out the unknown lengths to get it back.

Ioan had written this question. He showed how you could use the clues to work out the lengths of the sides. If the red circle was the second multiple of three, the length had to be 6cm.

Then, if the yellow star was half the length of the red circle, it’s length had to be 3cm.

Finny knew that in a rectilinear shape, the parallel sides have equal lengths. In this example, the blue heart would be the same length as it’s parallel sides. In his own words, “It’s a bit like a train track but one side has moved over to here…” Finny knew that the side with the red circle was 6cm and the other length parallel to the blue heart was 9cm, so the blue heart would be 6cm and 9cm added together.

Similarly, to work out the green triangle, he added the parallel sides, which were 3cm and 7cm, to give him a total of 10cm.

To check their answers, they looked at the final clue, which said green triangle is equal to 2/3 of blue heart. The blue heart was 15cm, so 1/3 of that would be 5cm and 2/3 would be 10cm. That was the answer Finny had written down for the green triangle, so he was confident with his answer.

DfES Outcomes for EYFS and National Curriculum (2013)

Numeracy Year 4 programme of study


  • measure and calculate the perimeter of a rectilinear figure (including squares) in centimetres and metres