The pattern
The second fraction is of one piece, not of the whole.

Something is cut in two, and one of the pieces is then cut again. The second fraction is never of the original pile: it is of the piece the question names — what was left after the first spend, or a part called out by name. Draw it as branches, and multiply along a branch to get that piece as a fraction of everything.
A second fraction arrives after the first and is attached to a piece rather than to the whole — 'of the remainder', 'of what was left', or 'of the ones that were red'.
Children take the second fraction of the original total instead of the piece it was named against, which makes the amount come out too big.
All the problem patterns → · the revision list
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.