Attention of reader one of old Russians of popular books is offered to on mathematics is a book of teacher of gymnasium Емельяна Ignatievich Игнатьева (1869-1923), confessed classic in area of pedagogics and entertaining mathematics. In the book the first edition of that went out in 1908, he collected plenty of various tasks on quick wits on arithmetic and geometry. For more than one hundred years popular mathematical the literature published in Russian language was strongly enriched, and now many tasks from the book of Игнатьева are well-known. And till today his works did not lose actuality and can serve as train aid.A book contains the tasks of entertaining character, having a different degree of difficulty. Tasks decide with bringing in of minimum information from arithmetic and geometry, but require quick wits and ability logically to think.Many tasks from a book "In the reign of shrewdness" entered the gold fund of materials of olympiads on mathematics for schoolchildren, university entrants and students of institutions of higher learning both in Soviet Union and in modern Russia.