Five semicircles on a line — the diameter
Five identical semicircles sit on one straight line — three above it, two below.
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65 worked examples are free without a code, two from every pattern. No account, either way.
The pattern behind it: Overlap and cancel
Two shapes share a region, and the shared part cancels out.
In this one. Cancel the shared overlap: what is left of two equal lengths must be equal too.
How to spot it
Shapes are joined, stacked, or have a piece cut off, and the question compares a perimeter or length before and after, rather than asking for an area from scratch.
Common mistake
Children assume perimeter changes the same way area does. Joining shapes or cutting off a corner can leave the perimeter unchanged, or change it by a different amount from the area.
The 33 problem patterns
All 33 patterns, one to a page — the 19 behind Primary 5 and 6 word problems, and the skills the younger years are built on. The bar model, how to spot it in the question, and the mistake children make. A booklet you can print. Free.
A PDF, and a note when new solutions go up.