Excel worksheet in which all 20 sums have been done incorrectly. Children are to explain the common error in each question. The aim is for children to demonstrate a deeper understanding of written methods. At an easier level, students can circle or highlight the error (or change its color if done on the computer). At a harder level, students can explain the common error made. Correctly working through the written method brings most of the errors to light. Very good for discussion, revising and l