Display the rectangle labelled only along the top (6 cm) and down one side (3 cm). Take three hands-up answers, not open call-outs. Listen for pupils who say the opposite sides must match.

6 by 3 rectangle: point to each side as you add, 6 + 3 + 6 + 3. Long sides match, short sides match. Perimeter 18 cm.

Square, side 5: it's a special rectangle, all four sides equal, so 4 lots of 5 = 20 cm.

7 by 2 rectangle: fill in the two missing sides first, then add, 7 + 2 + 7 + 2 = 18 cm.

Each side once, never twice. Ask which sides match before adding.

This round is for talking it through together — pupils take turns at the board and the class agrees or corrects out loud.

The interactive starts on a 4 cm by 3 cm rectangle. After the first pupil reads its perimeter, drag the rectangle's corner handles to re-set new given sides for the next pupil. Run four or five rectangles in turn (for example 4 by 3, then 5 by 2, then 6 by 4, then 3 by 3, then 8 by 2), each time asking the pupil at the board to fill in the matching opposite sides first, then read off the perimeter. Have the class confirm each opposite pair matches before the total is read. Watch for the common slip of measuring all four sides as if they were different.

Practice round: pupils take turns at the board, class confirms before moving on. Run the square and two rectangles briskly first. Then stop and model the L-shape fully on the board before any pupil tries it.

L-shape model: Sketch the L. Label known sides: bottom 6 cm, left 4 cm, both cut edges 2 cm. Hidden top = 6 − 2 = 4 cm. Hidden right = 4 − 2 = 2 cm. Point around once: 6 + 2 + 2 + 2 + 4 + 4 = 20 cm. Six sides, each once.

Slip to watch: adding only four outer sides, or counting a cut edge twice. Keep asking: which sides are hidden, and how do you find them? The 7 by 4 rectangle is new practice, give it a moment more.

Keep this brief. Tie the missing-side idea back to the L-shape in the challenge — finding hidden sides will matter again for more complex shapes.