This booklet, one of a series, has been developed for this project, A Porgram for Mathematically Underdeveloped Pupils. A project team, including inservice teachers, is being used to write and develop the materials for this program. The materials developed in this booklet are based on activities involving (1) similar geometric figures, (2) similar triangles, (3) classification of triangles, (4) constructing triangles and similar triangles, and (5) finding the missing length of similar polygons. (RP)