Summary This paper reports on the formalization in Mizar system [1], [2] of ten selected problems from W. Sierpinski’s book “250 Problems Elementary Number Theory” [7] (see [6] for details this concrete dataset). article is devoted mainly to arithmetic progressions: 52, 54, 55, 56, 60, 64, 70, 71, and 73 belong chapter “Arithmetic Progressions”, problem 50 “Relatively Prime Numbers”.