Excel worksheet in which every all 20 sums have been done incorrectly. Pupils are to explain the common error in each question. Aim is for pupils to demonstrate a deeper understanding of written methods (column, adding on, gelosia and bus shelter). At an easier level, pupils can circle or highlight the error (or change its colour if done on computer). At a harder level, pupils can explain the common error made. Working through the written method correctly brings most of the errors to light. Very good for discussion, revising and learning written methods, diagnosing issues and encouraging checking work - I tell the pupils ALL the errors have come from pupils’ work!