ABSTRACT
In this chapter, the author repeated his experiment in order to study the relationship between so-called inclusion problems and the problems of the “If I take all B away will there be any A left?” type. After the series of related problems many children discovered spontaneously the solution to the standard inclusion problems. In order to compare the percentages of spontaneous solutions of the inclusion problem in groups from different socioeconomic backgrounds we used the method of classroom testing and classroom teaching. The teaching part was just like an ordinary lesson, the teacher making use of the bright children to explain the problem to the others. The author discusses the amazingly good results of the taught children in the series of mixed problems. True, the experimenters warned the children beforehand to be careful and showed them examples of the different problems before actually testing them.
