Fold or cut a paper rectangle corner-to-corner in front of the class, then lay the two triangles on top of each other so pupils see they match exactly. Take three hands-up answers, not open call-outs. Hold out for the word half and, if a pupil offers it, the reason: the two triangles are identical, so each is one of two equal parts.

Work each one live; don't read the numbers in advance. Say squared units each time — area is cm², never plain cm.

First: base 8 cm, height 5 cm. Point to rectangle area 40 cm², then half — triangle 20 cm².

Second, right-angled: base 6 cm, height 4 cm. Rectangle 24 cm², triangle half 12 cm². Easiest to see as half a rectangle — the two shorter sides ARE base and height.

Third, leaning: base 10 cm, height 6 cm. Apex is offset so the slant is clearly longer than the straight-up height. Build the halving live — flip a matching copy in to fill the rest, so pupils SEE two equal triangles fill the whole 60 cm² rectangle; this triangle is 30 cm².

The slip: pupils reach for the slanted length. Ask which line meets the base with a right angle? before revealing. Point to the slant, then to the straight-up line.

Talk this one through together — pupils take turns at the board and the class agrees or corrects out loud.

Resize the interactive to each set of dimensions in turn and let a pupil read the live area off the tool, then have the class confirm it by hand: rectangle area, then half. Exact resize targets: (1) base 8 cm, height 3 cm; (2) base 12 cm, height 5 cm; (3) base 10 cm, height 6 cm with the apex dragged to one side so the tip leans. Each time, ask the pupil to point to the perpendicular height before they compute.

Watch for anyone using the slanted side on the leaning triangle. Revoice a good answer: so the height is the straight-up distance, whatever way the tip leans.

Model the reverse move slowly on the board: read the on-screen working aloud — area 20 cm², double it to get the whole rectangle back (40 cm²), then divide by the base (40 ÷ 8 = 5 cm). Set the tool to base 8 cm, height 5 cm and check the area comes back to 20 cm². Have one pupil re-explain in their own words why we double first before you move on — this is the move the challenge reuses.

If the interactive is unavailable: draw each triangle inside its rectangle on squared paper or on the board and work the same numbers by hand, including the reverse example.

This is the practice round — pupils take turns at the board, check each answer, and the class confirms before moving on. Every move here has already been modelled in Try It Together, so keep the board work brisk rather than over-explaining.

Resize targets: (1) base 10 cm, height 6 cm; (2) base 7 cm, height 8 cm; (3) base 8 cm, height 5 cm with the apex offset so it leans (the config already renders it leaning). The third checks that pupils use the perpendicular height even when the tip leans and the slanted side looks tempting.

The last one reuses the reverse move from Try It Together: whole rectangle area is 2 × 30 = 60 cm², so height = 60 ÷ 12 = 5 cm — hold out for a pupil to explain they doubled the area first. This is the one that lands the whole rule.

If the interactive is unavailable: sketch each triangle on squared paper or on the board and work the numbers by hand, including doubling the area for the reverse puzzle.