The book is devoted to a deeper study of both standard concepts of descriptive statistics studied in high school (average, median, variance, histogram) and related concepts (distribution function, quartiles, etc.). Moreover, descriptive statistics are presented as a mathematical science with definitions, theorems, strict proofs, using the usual algebraic language of formulas. The theory is illustrated by a large number of problems. The book is based on the experience of teaching statistics in UK schools. The presented theoretical facts and their simple applications will be useful for junior students of universities who are starting to study statistics and probability theory, and teachers of mathematics. The book can be studied in grade 9, in classes with in-depth study of mathematics.